[TG3]: Support shutdown WoL.

Support WoL during shutdown by calling
tg3_set_power_state(tp, PCI_D3hot) during tg3_close().

Change the power state parameter to pci_power_t type and use
constants defined in pci.h.

Certain ethtool operations cannot be performed after tg3_close()
because the device will go to low power state. Add return -EAGAIN
in such cases where appropriate.

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