tg3: Use pci_dev pm_cap

Use the already existing pm_cap variable in struct pci_dev for
determining the power management offset.  This saves the driver from
having to keep track of an extra variable.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Cc: Nithin Nayak Sujir <nsujir@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>