mtd: mtdoops: do not use mtd->panic_write directly

Instead of checking if 'mtd->panic_write' is defined, call 'mtd_panic_write()'
and check the error code - '-EOPNOTSUPP' will be returned if the function is
not defined.

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 2c2a922..b729640 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -311,6 +311,8 @@
 				  size_t *retlen, const u_char *buf)
 {
 	*retlen = 0;
+	if (!mtd->panic_write)
+		return -EOPNOTSUPP;
 	return mtd->panic_write(mtd, to, len, retlen, buf);
 }