tg3: Skip powering down function 0 on certain serdes devices

On the 5718, 5719 and 5720 serdes devices, powering down function 0
results in all the other ports being powered down. Add code to skip
function 0 power down.

v2:
 - Modify tg3_phy_power_bug() function to use a switch instead of a
   complicated if statement. Suggested by Joe Perches.

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