[media] v4l2-ctrls: don't initially set CH_VALUE for write-only controls

When sending the SEND_INITIAL event for write-only controls the
V4L2_EVENT_CTRL_CH_VALUE flag should not be set. It's meaningless.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>