V4L/DVB (3254): Don't reprogram the video standard if it is unchanged.

- Don't reprogram the video standard if the new standard
equals the old standard.

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