[TG3]: Walk PCI capability lists.

Newer tg3 devices shuffle around the registers in PCI configuration
space.  This patch changes the way the driver accesses the PCI
capabilities registers.  Hardcoded register locations are replaced with
offsets from pci_find_capability() return values.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>