mtd: spi-nor: core: Use container_of to get the pointer to struct spi_nor

"struct mtd_info mtd" is member of "struct spi_nor", there's no need
to use "mtd->priv". Get the pointer to the containing struct spi_nor
by using container_of. While here, make the function inline and
get rid of the __maybe_unused.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20211029172633.886453-3-tudor.ambarus@microchip.com

drivers/mtd/spi-nor/core.c

@@ -3134,7 +3134,6 @@ int spi_nor_scan(struct spi_nor *nor, const char *name,
 
 	if (!mtd->name)
 		mtd->name = dev_name(dev);
-	mtd->priv = nor;
 	mtd->type = MTD_NORFLASH;
 	mtd->writesize = nor->params->writesize;
 	mtd->flags = MTD_CAP_NORFLASH;

drivers/mtd/spi-nor/core.h

@@ -551,9 +551,9 @@ void spi_nor_try_unlock_all(struct spi_nor *nor);
 void spi_nor_register_locking_ops(struct spi_nor *nor);
 void spi_nor_otp_init(struct spi_nor *nor);
 
-static struct spi_nor __maybe_unused *mtd_to_spi_nor(struct mtd_info *mtd)
+static inline struct spi_nor *mtd_to_spi_nor(struct mtd_info *mtd)
 {
-	return mtd->priv;
+	return container_of(mtd, struct spi_nor, mtd);
 }
 
 #endif /* __LINUX_MTD_SPI_NOR_INTERNAL_H */