staging: vboxvideo: fix mode_valid's return type

The method struct drm_connector_helper_funcs::mode_valid is defined
as returning an 'enum drm_mode_status' but the driver implementation
for this method uses an 'int' for it.

Fix this by using 'enum drm_mode_status' in the driver too.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vboxvideo/vbox_mode.c

@@ -573,7 +573,7 @@ static int vbox_get_modes(struct drm_connector *connector)
 	return num_modes;
 }
 
-static int vbox_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status vbox_mode_valid(struct drm_connector *connector,
 			   struct drm_display_mode *mode)
 {
 	return MODE_OK;