[media] soc-camera: Don't fail at module init time if no device is present

The soc-camera module exports functions that are needed by soc-camera
client drivers even when not running in soc-camera mode. Replace the
platform_driver_probe() with a platform_driver_register() call to avoid
module load failures if no soc-camera device is present.

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