ssb: use pci_dev->revision

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

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>