drm/i2c: tda998x: 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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424131522.2460-1-luc.vanoostenryck@gmail.com

drivers/gpu/drm/i2c/tda998x_drv.c

@@ -1106,7 +1106,7 @@ static int tda998x_connector_get_modes(struct drm_connector *connector)
 	return n;
 }
 
-static int tda998x_connector_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status tda998x_connector_mode_valid(struct drm_connector *connector,
 					struct drm_display_mode *mode)
 {
 	/* TDA19988 dotclock can go up to 165MHz */