V4L/DVB: saa7146: fix up bytesperline if it is an impossible value

xawtv using DGA on a Radeon graphics card provides bogus
values to S_FBUF, which will then screw up overlay video:

https://bugs.launchpad.net/ubuntu/+source/xawtv/+bug/499734



This fixes the bytesperline value if it is off completely.

Signed-off-by: Michael Hunold <michael@mihu.de>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>