mtd: introduce mtd_has_oob helper

We are working in the direction of making sure that MTD clients to not
use 'mtd->func' pointers directly. In some places we want to know if
OOB operations are supported by an MTD device. Introduce 'mtd_has_oob()'
helper for these purposes.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/sm_ftl.c b/drivers/mtd/sm_ftl.c
index 4ec2af7..072ed59 100644
--- a/drivers/mtd/sm_ftl.c
+++ b/drivers/mtd/sm_ftl.c
@@ -645,8 +645,8 @@
 	if (!ftl->smallpagenand && mtd->oobsize < SM_OOB_SIZE)
 		return -ENODEV;
 
-	/* We use these functions for IO */
-	if (!mtd->read_oob || !mtd->write_oob)
+	/* We use OOB */
+	if (!mtd_has_oob(mtd))
 		return -ENODEV;
 
 	/* Find geometry information */