[media] tc358743: allow event subscription

This is useful to subscribe to HDMI hotplug events via the
V4L2_CID_DV_RX_POWER_PRESENT control.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>