[media] v4l2: pass std by value to the write-only s_std ioctl

This ioctl is defined as IOW, so pass the argument by value instead of by
reference. I could have chosen to add const instead, but this is 1) easier
to handle in drivers and 2) consistent with the s_std subdev operation.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>