V4L/DVB (7680): pvrusb2-dvb: add pvr2_dvb_bus_ctrl to allow frontends to negotiate bus access

This function is just a skeleton for now -
a placeholder to remind us to fix it.

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