ar71xx: fix invalid pointer dereference in rb95x_nand_scan_fixup()

ar71xx: fix invalid pointer dereference in rb95x_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
chip->options causes an invalid pointer dereference (FS#1200).

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

Fixes: 7bbf4117c6fe ("ar71xx: Add kernel 4.9 support")
Signed-off-by: Gabor Juhos <juhosg@freemail.hu>