cmd: pinmux: update result of do_status

Update the result of do_status and always returns a CMD_RET_ value
(-ENOSYS was a possible result of show_pinmux).

This patch also adds pincontrol name in error messages (dev->name)
and treats correctly the status sub command when pin-controller device is
not selected.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_pinmux.py b/test/py/tests/test_pinmux.py
index 0cbbae0..b3ae2ab 100644
--- a/test/py/tests/test_pinmux.py
+++ b/test/py/tests/test_pinmux.py
@@ -13,9 +13,9 @@
 @pytest.mark.buildconfigspec('cmd_pinmux')
 def test_pinmux_usage_2(u_boot_console):
     """Test that 'pinmux status' executed without previous "pinmux dev"
-    command displays pinmux usage."""
+    command displays error message."""
     output = u_boot_console.run_command('pinmux status')
-    assert 'Usage:' in output
+    assert 'pin-controller device not selected' in output
 
 @pytest.mark.buildconfigspec('cmd_pinmux')
 @pytest.mark.boardspec('sandbox')