mips: nand: make use of mtd_to_nand() where appropriate

mtd_to_nand() was recently introduced to avoid direct accesses to the
mtd->priv field. Update all MIPS specific implementations to use this
helper.

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