tg3: Avoid tx lockups on 5755+ devices

In certain edge conditions, internal tx resources can get corrupted.
This patch enables a bit that will fix the problem.

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>