media: ov7740: Set subdev HAS_EVENT flag

The driver has event support implemented but fails to set the flag
enabling event support. Set the flag to enable control events.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>