mtd: nand: add helpers to access ->priv

Add two helpers to access the field reserved for private controller data.
This makes it clearer what this field is reserved for and ease future
refactoring.

Signed-off-by: Boris Brezillon <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

include/linux/mtd/nand.h

···
 	return &chip->mtd;
 }
 
+static inline void *nand_get_controller_data(struct nand_chip *chip)
+{
+	return chip->priv;
+}
+
+static inline void nand_set_controller_data(struct nand_chip *chip, void *priv)
+{
+	chip->priv = priv;
+}
+
 /*
  * NAND Flash Manufacturer ID Codes
  */