ar71xx: restore rb4xx kernel partition size to the original value

The new lzma compressed elf kernel image fits into
that, even on devices with large page NAND chips.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 34274

target/linux/ar71xx/files/drivers/mtd/nand/rb4xx_nand.c

@@ -62,7 +62,7 @@ static struct mtd_partition rb4xx_nand_partitions[] = {
 	{
 		.name	= "kernel",
 		.offset	= (256 * 1024),
-		.size	= (6 * 1024 * 1024) - (256 * 1024),
+		.size	= (4 * 1024 * 1024) - (256 * 1024),
 	},
 	{
 		.name	= "rootfs",