Any V4L2 subdevice that implements controls and declares
V4L2_SUBDEV_FL_HAS_DEVNODE should also declare V4L2_SUBDEV_FL_HAS_EVENTS
and implement subscribe_event and unsubscribe_event hooks.

This driver didn't and would therefore fail v4l2-compliance
testing.

Add the relevant hooks.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>