tg3: Enable phy APD for 5717 and later asic revs

This patch enables the gphy autopowerdown feature in the phy for all
new devices that support it.

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