[media] V4L: sh_mobile_csi2: do not guess the client, the host tells us

We do not have to scan the list of subdevices to find our client - the
sensor, the host has already set our grp_id value.

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