V4L/DVB (6313): ivtv: Replace list_for_each+list_entry with list_for_each_entry

Also fixed a few cases of cut&paste errors where 'buf' would be set to the
first entry in the list prior to be used as the loop iterator.  In one case
the value of buf was used before it was changed, but the rest were
unnecessary.

There was one list_for_each+list_entry loop that wasn't changed, since it
depending on the loop iterator being left as NULL if the list was empty.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
CC: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>