cmd: Fix an error code in cmd_mux_find()

This returns the wrong variable.  It ends up returning NULL when it was
suppose to return an error pointer.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>

cmd/mux.c

@@ -49,7 +49,7 @@ static struct mux_control *cmd_mux_find(char *const argv[])
 
 	chip = dev_get_uclass_priv(dev);
 	if (!chip)
-		return ERR_PTR(ret);
+		return ERR_PTR(-EINVAL);
 
 	if (id >= chip->controllers)
 		return ERR_PTR(-EINVAL);