V4L/DVB: dvb/bt8xx: Clean-up init and exit functions

The init and exit functions are needlessly complex. Remove the bloat:
* Drop irrelevant/outdated comments.
* Remove useless bt878_pci_driver_registered global variable.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mauro Carvalho chehab <mchehab@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>