[media] ivtv: improve subscribe_event handling

Simplify logic and call v4l2_ctrl_subscribe_event() directly instead
of copying its content over to ivtv_subscribe_event().

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>