[TG3]: Add ASPM workaround.

This patch adds workaround to fix performance problems caused by slow
PCIE L1->L0 transitions on ICH8 platforms.

Changed all magic numbers to constants as suggested by Jeff Garzik.

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>