ar71xx: fix invalid pointer dereference in c60_nand_scan_fixup()

Since Linux 4.6, mtd->priv no longer points to the NAND specific
structure. Under 4.9 it contains NULL, thus using it to access
the fields of the nand_chip structure causes an invalid pointer
dereference.

Update the code to use the mtd_to_nand() helper under 4.9 to obtain
the address of the chip specific data.

Compile tested only.

Fixes: 7bbf4117c6 ("ar71xx: Add kernel 4.9 support")
Signed-off-by: Gabor Juhos <juhosg@freemail.hu>
Tested-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/ar71xx/files/arch/mips/ath79/mach-c60.c

@@ -180,7 +180,11 @@ static const struct mtd_ooblayout_ops c60_nand_ecclayout_ops = {
 
 static int c60_nand_scan_fixup(struct mtd_info *mtd)
 {
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,6,0)
 	struct nand_chip *chip = mtd->priv;
+#else
+	struct nand_chip *chip = mtd_to_nand(mtd);
+#endif
 
 	chip->ecc.size = 512;
 	chip->ecc.strength = 4;