[media] bt8xx: use pci_dev->subsystem_{vendor|device}

The driver reads PCI subsystem IDs from the PCI configuration registers while
they are already stored by the PCI subsystem in the 'subsystem_{vendor|device}'
fields of 'struct pci_dev'...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>