V4L/DVB (4377): Force horizontal resolution limits in the pvrusb2 driver

This change causes the pvrusb2 driver to limit horizontal resolution
to be only 720 if 24xxx device hardware is being used.  This is a
workaround for an unsolved problem in the driver where if the
resolution is set to something other than 720 the video quality will
be _seriously_ degraded.  It only happens on 24xxx devices (29xxx are
unaffected by the problem and unaffected by this change).  Once the
problem is finally solved, this small change can be removed.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>