IB/qib: Use pci_dev->revision

The driver reads PCI revision ID from the PCI configuration register
while it's already stored by PCI subsystem in the revision field of
struct pci_dev.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>