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>
diff --git a/arch/mips/alchemy/devboards/db1300.c b/arch/mips/alchemy/devboards/db1300.c
index b580770..d3c087f 100644
--- a/arch/mips/alchemy/devboards/db1300.c
+++ b/arch/mips/alchemy/devboards/db1300.c
@@ -150,7 +150,7 @@
 static void au1300_nand_cmd_ctrl(struct mtd_info *mtd, int cmd,
 				 unsigned int ctrl)
 {
-	struct nand_chip *this = mtd->priv;
+	struct nand_chip *this = mtd_to_nand(mtd);
 	unsigned long ioaddr = (unsigned long)this->IO_ADDR_W;
 
 	ioaddr &= 0xffffff00;