tg3: Don't turn off led on 5719 serdes port 0

Turning off led on port 0 of the 5719 serdes causes all other ports to
lose power and stop functioning. Add tg3_phy_led_bug() function to check
for this condition. We use a switch() in tg3_phy_led_bug() for
consistency with the tg3_phy_power_bug() function.

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