mtd: introduce mtd_write interface

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 83a8437..f8aac4b 100644
--- a/drivers/mtd/tests/mtd_stresstest.c
+++ b/drivers/mtd/tests/mtd_stresstest.c
@@ -192,7 +192,7 @@
 		}
 	}
 	addr = eb * mtd->erasesize + offs;
-	err = mtd->write(mtd, addr, len, &written, writebuf);
+	err = mtd_write(mtd, addr, len, &written, writebuf);
 	if (unlikely(err || written != len)) {
 		printk(PRINT_PREF "error: write failed at 0x%llx\n",
 		       (long long)addr);