tg3 / broadcom: Refine AC131 APD support

Auto power-down (APD) support is a power-saving feature.  It should be
selectively enabled since it might expose MAC bugs.  This patch changes
the code to enable APD only if the PHY_BRCM_AUTO_PWRDWN_ENABLE flag is
set.  The tg3 driver was changed to set this bit.

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