mlx4: use pci_dev->revision

Commit 725c8999 (mlx4_en: Reporting HW revision
in ethtool -i) added code to read the revision ID from the PCI configuration
register while it's already stored by PCI subsystem in the 'revision' field of
'struct pci_dev'...

While at it, move the code being changed a bit in order to not break the
initialization sequence.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>