V4L/DVB (3292): Fix signed/unsigned bug in brightness handling of cx25840

- Fix signed/unsigned bug in brightness handling (set to 0 and 128 was returned).

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