[media] gspca_pac7311: Switch to coarse expo autogain algorithm

We can only control the clockdivider to control exposure on the pac7311,
making our expo control coarse, switch to an autogain algorithm optimized for
this.

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