tg3: remove unnecessary read of PCI_CAP_ID_EXP

The PCIE capability offset is saved during PCI bus walking.  Use the
value from pci_dev instead of checking in the driver and saving it off
the the driver specific structure.  It will remove an unnecessary search
in the PCI configuration space if this value is referenced instead of
reacquiring it.

v2 of the patch re-adds the PCI_EXPRESS flag and adds comments
describing why it is necessary.

[ pdev->pcie_cap --> pci_pcie_cap(pdev) -DaveM ]

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: David S. Miller <davem@davemloft.net>