media: ipu3-cio2: Set CSI-2 receiver sub-device entity function

Set the entity function for the four CSI-2 receiver sub-devices the driver
creates. This avoids a kernel warning from each as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>