[media] em28xx: std fixes: don't implement in webcam mode, and fix std changes

When in webcam mode the STD API shouldn't be implemented.
When changing the standard the resolution wasn't updated, and there was no
check against streaming-in-progress.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>