staging: mt29f_spinand: make use of mtd_to_nand()

mtd_to_nand() was recently introduced to avoid direct accesses to the
mtd->priv field. Use it where appropriate.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>