tg3: Avoid setting power.can_wakeup for devices that cannot wake up

The tg3 driver uses device_init_wakeup() in such a way that the
device's power.can_wakeup flag may be set even though the PCI
subsystem cleared it before, in which case the device cannot wake
up the system from sleep states.  Modify the driver to only change
the power.can_wakeup flag if the device is not capable of generating
wakeup signals.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>