V4L/DVB (6314): saa7134: Replace list_for_each+list_entry with list_for_each_entry

A couple loops weren't changed because they expected the loop iterator
to be left as NULL if the list was empty.  Maybe the code should just
check for that first, then loop?

Adjust some of the loop logic to be simpler.

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