[media] cx18: add support for control events

v4l2-compliance failed due to missing control event support in cx18.
Add this to the driver.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>