mtd: mtdcore: Fix build warning when CONFIG_MTD_CHAR is not defined

Fix the following build warning:

drivers/mtd/mtdcore.c: In function ‘mtd_release’:
drivers/mtd/mtdcore.c:110: warning: unused variable ‘mtd’

This happens when neither CONFIG_MTD_CHAR nor CONFIG_MTD_CHAR_MODULE are defined.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/mtdcore.c

@@ -107,7 +107,7 @@ static LIST_HEAD(mtd_notifiers);
  */
 static void mtd_release(struct device *dev)
 {
-	struct mtd_info *mtd = dev_get_drvdata(dev);
+	struct mtd_info __maybe_unused *mtd = dev_get_drvdata(dev);
 	dev_t index = MTD_DEVT(mtd->index);
 
 	/* remove /dev/mtdXro node if needed */