tg3: Do not use legacy PCI power management

The tg3 driver uses the legacy PCI power management, so it has to do
some PCI-specific things in its ->suspend() and ->resume() callbacks,
which isn't necessary and should better be done by the PCI
sybsystem-level power management code.

Convert tg3 to the new PCI power management framework and make it
let the PCI subsystem take care of all the PCI-specific aspects of
device handling during system power transitions.

Tested on HP nx6325 with a NetXtreme BCM5788 adapter.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>