V4L/DVB (8757): v4l-dvb: fix a bunch of sparse warnings

Fixed a lot of sparse warnings: mostly warnings about shadowed variables
and signed/unsigned mismatches.

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