V4L/DVB: gspca - ov534: Add Auto Exposure

This also makes manual exposure actually work: it never worked before
because AEC was always enabled.

Signed-off-by: Max Thrun <bear24rw@gmail.com>
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>