tg3: implement ethtool set_phys_id

Implement control of LED via set_phys_id.
Note: since PHY is powered off if device is down, this board
only allows blinking if device is up.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>