[media] gspca_pac7311: Make sure exposure changes get applied immediately

It turns out that the flush to sensor command needs to be done per register
bank. We were missing one such flush in set_exposure, causing exposure changes
to only show up when another setting in the same bank also got changed.

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