ar71xx: fix invalid pointer dereference in c60_nand_scan_fixup()

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: 7bbf4117c6fe ("ar71xx: Add kernel 4.9 support")
Signed-off-by: Gabor Juhos <juhosg@freemail.hu>
Tested-by: Christian Lamparter <chunkeey@gmail.com>