[media] gspca_pac7302: Document some more registers

Note like all info on the pac73xx chips, this info was found by trial and
error, so it is not necessarily 100% correct.

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