mac80211: Fix build on mpc85xx target

This fixes the following compile error seen on the mpc85xx target:
  CC [M]  /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.o
In file included from /builder/shared-workdir/build/staging_dir/toolchain-powerpc_8540_gcc-8.4.0_musl/include/stddef.h:17,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/include/uapi/linux/wireless.h:77,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/include/linux/wireless.h:13,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.c:89:
/builder/shared-workdir/build/staging_dir/toolchain-powerpc_8540_gcc-8.4.0_musl/include/bits/alltypes.h:106:15: error: conflicting types for 'ptrdiff_t'
 typedef _Addr ptrdiff_t;
               ^~~~~~~~~
In file included from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/types.h:4,
                 from ./include/linux/list.h:5,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/list.h:3,
                 from ./include/linux/module.h:9,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/module.h:3,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.c:79:
./include/linux/types.h:65:28: note: previous declaration of 'ptrdiff_t' was here
 typedef __kernel_ptrdiff_t ptrdiff_t;
                            ^~~~~~~~~
scripts/Makefile.build:265: recipe for target '/linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.o' failed

Fixes: d6b158b869 ("mac80211: Update to 4.19.137-1")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 04b1a11f5c)

package/kernel/mac80211/Makefile

@@ -11,7 +11,7 @@ include $(INCLUDE_DIR)/kernel.mk
 PKG_NAME:=mac80211
 
 PKG_VERSION:=4.19.137-1
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_SOURCE_URL:=@KERNEL/linux/kernel/projects/backports/stable/v4.19.137/
 PKG_HASH:=dc5eea4f77fc5c43b69e38f46fbf766880fa4bdeef83dcc8dcc85aa6b645bb7c
 

package/kernel/mac80211/patches/subsys/090-wireless-Use-linux-stddef.h-instead-of-stddef.h.patch

@@ -0,0 +1,33 @@
+From 1b9ae0c92925ac40489be526d67d0010d0724ce0 Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <hauke@hauke-m.de>
+Date: Thu, 21 May 2020 22:14:22 +0200
+Subject: [PATCH] wireless: Use linux/stddef.h instead of stddef.h
+
+When compiling inside the kernel include linux/stddef.h instead of
+stddef.h. When I compile this header file in backports for power PC I
+run into a conflict with ptrdiff_t. I was unable to reproduce this in
+mainline kernel. I still would like to fix this problem in the kernel.
+
+Fixes: 6989310f5d43 ("wireless: Use offsetof instead of custom macro.")
+Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
+Link: https://lore.kernel.org/r/20200521201422.16493-1-hauke@hauke-m.de
+Signed-off-by: Johannes Berg <johannes.berg@intel.com>
+---
+ include/uapi/linux/wireless.h | 6 +++++-
+ 1 file changed, 5 insertions(+), 1 deletion(-)
+
+--- a/include/uapi/linux/wireless.h
++++ b/include/uapi/linux/wireless.h
+@@ -74,7 +74,11 @@
+ #include <linux/socket.h>		/* for "struct sockaddr" et al	*/
+ #include <linux/if.h>			/* for IFNAMSIZ and co... */
+ 
+-#include <stddef.h>                     /* for offsetof */
++#ifdef __KERNEL__
++#	include <linux/stddef.h>	/* for offsetof */
++#else
++#	include <stddef.h>		/* for offsetof */
++#endif
+ 
+ /***************************** VERSION *****************************/
+ /*