V4L/DVB (10813): v4l2: New function v4l2_video_std_frame_period

Some code was calling v4l2_video_std_construct() when all it cared about
was the frame period.  So make a function that just returns that and have
v4l2_video_std_construct() use it.

At this point there are no users of v4l2_video_std_construct() left outside
of v4l2-ioctl, so it could be un-exported and made static.

Change v4l2_video_std_construct() so that it doesn't zero out the struct
v4l2_standard passed in.  It's already been zeroed out in the common ioctl
code.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>