cmd: mtd: Prevent use of uninitialised variable

ret maybe used uninitialised in some cases so instead
initialise it first to prevent this.

This issue was found by Smatch.

Fixes: 9671243e8d (cmd: mtd: Use the subcmd infrastructure to declare mtd sub-commands)
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>

cmd/mtd.c

@@ -477,7 +477,7 @@ static int do_mtd_io(struct cmd_tbl *cmdtp, int flag, int argc,
 	struct mtd_info *mtd;
 	u32 oob_len;
 	u8 *buf;
-	int ret;
+	int ret = CMD_RET_SUCCESS;
 
 	if (argc < 2)
 		return CMD_RET_USAGE;