mtd: do not use mtd->get_unmapped_area directly

Remove direct usage of mtd->get_unmapped_area. Instead, just call
'mtd_get_unmapped_area()' which will return -EOPNOTSUPP if the function
is not implemented and test for this error code.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index b355a83..2c2a922 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -280,6 +280,8 @@
 						  unsigned long offset,
 						  unsigned long flags)
 {
+	if (!mtd->get_unmapped_area)
+		return -EOPNOTSUPP;
 	return mtd->get_unmapped_area(mtd, len, offset, flags);
 }