V4L/DVB (11912): em28xx: Use v4l bounding/alignment function

The v4l function has a better algorithm for aligning image size.

It appears that the em2800 can only scale by 50% or 100%, i.e. the only
heights supported might be 240 and 480.  In that case the old code would
set any height other than 240 to 480.  Request 240 get 240, but request 239
and then you get 480.  Change it to round to the nearest supported value.

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