[media] vivid: improve subscribe event handling

We already check for the V4L2_EVENT_CTRL inside
v4l2_ctrl_subscribe_event() so just move this fuction to the default:
branch of the switch and let it does the job for us.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>