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>
diff --git a/cmd/mux.c b/cmd/mux.c
index 833266f..c75907a 100644
--- a/cmd/mux.c
+++ b/cmd/mux.c
@@ -49,7 +49,7 @@
 
 	chip = dev_get_uclass_priv(dev);
 	if (!chip)
-		return ERR_PTR(ret);
+		return ERR_PTR(-EINVAL);
 
 	if (id >= chip->controllers)
 		return ERR_PTR(-EINVAL);