media: usbtv: fix brightness and contrast controls

media: usbtv: fix brightness and contrast controls

Because the brightness and contrast controls share a register,
usbtv_s_ctrl needs to read the existing values for both controls before
inserting the new value. However, the code accidentally wrote to the
registers (from an uninitialised stack array), rather than reading them.

The user-visible effect of this was that adjusting the brightness would
also set the contrast to a random value, and vice versa -- so it wasn't
possible to correctly adjust the brightness of usbtv's video output.

Tested with an "EasyDAY" UTV007 device.

Fixes: 7ff7805ce700 ("usbtv: add video controls")
Signed-off-by: Adam Sampson <ats@offog.org>
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>