mtd: nand: add an mtd_to_nand() helper

Some drivers are retrieving the nand_chip pointer using the container_of
macro on a struct wrapping both the nand_chip and the mtd_info struct while
the standard way of retrieving this pointer is through mtd->priv.
Provide an helper to do that.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/linux/mtd/nand.h

@@ -719,6 +719,11 @@ struct nand_chip {
 	void *priv;
 };
 
+static inline struct nand_chip *mtd_to_nand(struct mtd_info *mtd)
+{
+	return mtd->priv;
+}
+
 /*
  * NAND Flash Manufacturer ID Codes
  */