iwlwifi: use pci_dev->revision, again

Commit ff938e43 (net: use pci_dev->revision,
again) already converted this driver to using the 'revision' field of 'struct
pci_dev' but commit 084dd791 (iwlagn: move PCI
related operations from probe and remove to PCI layer) has again added the code
to read the PCI revision ID register...

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