iwlegacy: use pci_dev->revision

Commit be663ab6 (iwlwifi: split the drivers for
agn and legacy devices 3945/4965) added code to read the 4965's 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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>