V4L/DVB (12948): v4l1-compat: fix VIDIOC_G_STD handling

The VIDIOC_G_STD ioctl may not be present in the case of radio receivers.
In that case G_STD will return an error. The v4l1-compat layer should not
attempt to propagate that error to the caller, instead it should be
ignored.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>