As noted in the headers for V4L2_SUBDEV_FL_HAS_EVENTS,
"controls can send events, thus drivers exposing controls
should set this flag".

This driver exposes controls, but didn't reflect that it
could generate events. Correct this, and add the default
event handler functions.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>