V4L/DVB: gspca_pac7302: much improved exposure control

My experience with fixing up the controls for the PAS sensors in
sonixb, has lead me to re-investigate the exposure control for the
pac7302. I've now found a regular exposure register in register bank 3
(which seems to be the sensor registers bank), and with this added a proper
fine grained exposure control. This patch also updates the do_autogain
function to work properly with this new finer grained control.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>