tg3: Fix 5717 serdes powerdown problem

If port 0 of a 5717 serdes device powers down, it hides the phy from
port 1.  This patch works around the problem by keeping port 0's phy
powered up.

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