linux: generic: fix split_squashfs for 3.6

_read expects the retlen to be initialized to zero, which the mtd_read
helper does. So fix it by using mtd_read instead of calling _read directly.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34093 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/generic/patches-3.6/400-rootfs_split.patch

@@ -55,7 +55,7 @@
 +	struct squashfs_super_block sb;
 +	int len, ret;
 +
-+	ret = master->_read(master, offset, sizeof(sb), &len, (void *) &sb);
++	ret = mtd_read(master, offset, sizeof(sb), &len, (void *) &sb);
 +	if (ret || (len != sizeof(sb))) {
 +		printk(KERN_ALERT "split_squashfs: error occured while reading "
 +			"from \"%s\"\n", master->name);