forcedeth: Do not use legacy PCI power management

The forcedeth driver uses the legacy PCI power management, so it has
to do PCI-specific things in its ->suspend() and ->resume() callbacks
and some of them are not done correctly.

Convert forcedeth 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 with nVidia Corporation MCP55 Ethernet (rev a2).

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