[media] gspca_ov534: make AGC and AWB controls independent

Even if the best results are indeed achieved with both AGC and AWB
enabled, the webcam is capable of setting these independently, and the
user can see the difference of any of the 4 combinations of these two
boolean controls.

Removing the dependency from one another simplifies the code and gives
more control to the user.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>