V4L/DVB: gspca - ov534: Fix and document setting manual exposure

Document that even if the state is a u8 value, both MSB and LSB are set
as sd->exposure represents half of the value we are going to set into
registers.

Skip setting exposure when AEC is enabled.

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>