V4L/DVB (7692): pvrusb2-dvb: Further clean up dvb init/tear-down

Move pvr2_dvb_adapter usage out of the pvrusb2 driver core - it's
really private to the pvrusb2-dvb module and nothing outside of the
dvb implementation should care about it.  Creation / destruction of
the pvr2_dvb_adapter instance is now contained entirely within
pvrusb2-dvb.c.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>