mtd: introduce mtd_can_have_bb helper

This patch introduces new 'mtd_can_have_bb()' helper function which checks
whether the flash can have bad eraseblocks. Then it changes all the
direct 'mtd->block_isbad' use cases with 'mtd_can_have_bb()'.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/tests/mtd_stresstest.c b/drivers/mtd/tests/mtd_stresstest.c
index 399aa2b..7b33f22 100644
--- a/drivers/mtd/tests/mtd_stresstest.c
+++ b/drivers/mtd/tests/mtd_stresstest.c
@@ -227,8 +227,7 @@
 		return -ENOMEM;
 	}
 
-	/* NOR flash does not implement block_isbad */
-	if (mtd->block_isbad == NULL)
+	if (!mtd_can_have_bb(mtd))
 		return 0;
 
 	printk(PRINT_PREF "scanning for bad eraseblocks\n");