[media] soc_camera: remove use of current_norm

The current_norm field is deprecated, so don't set it. Since it is set to
V4L2_STD_UNKNOWN which is 0 it didn't do anything anyway.
Also remove a few other unnecessary uses of V4L2_STD_UNKNOWN.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>