V4L/DVB (7241): cx25840: code cleanup

- Use min()
- Eliminate extraneous variables

Signed-off-by: Tyler Trafford <ttrafford@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>