V4L/DVB (12580): soc-camera: remove now unneeded subdevice group ID assignments

Since we are not using v4l2_device_call_* calls any more, we don't need to
initialise subdevice .grp_id any more. This also fixes compiler warnings on
64-bit platforms.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>