btrfs-progs: use the correct variable

It's res instead of ret, wrong error message could be prointed in case
of error.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

cmds-device.c

@@ -183,7 +183,7 @@ static int cmd_rm_dev(int argc, char **argv)
 		if (res) {
 			const char *msg;
 
-			if (ret > 0)
+			if (res > 0)
 				msg = btrfs_err_str(res);
 			else
 				msg = strerror(e);