r8169: 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.  Also, it will remove an unnecessary
search in the PCI configuration space if this value is referenced
instead of reacquiring it.

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