mtd: do not use mtd->sync directly

This patch teaches 'mtd_sync()' to do nothing when the MTD driver does
not have the '->sync()' method, which allows us to remove all direct
'mtd->sync' accesses.

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 a58ecf4..305f12b 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -399,7 +399,8 @@
 
 static inline void mtd_sync(struct mtd_info *mtd)
 {
-	mtd->sync(mtd);
+	if (mtd->sync)
+		mtd->sync(mtd);
 }
 
 /* Chip-supported device locking */