[media] soc-camera: fix a recent multi-camera breakage on sh-mobile

With the introduction of CSI2 support on sh-mobile, the host driver
switched to using v4l2_device_call_until_err() with grp_id == 0 to
call subdev operations on the sensor and the CSI2 subdev. However,
this has broken multi-client set ups like the one on migor, because
that way all operations get called on both clients. To fix this add
a grp_id and set it to the client private context.

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