bnx2x: Fix chip hanging due to TX pipe stall.

Fix a problem in which the 578xx chip hangs after running traffic, and then pulling the network cable.
This occurs since TX pipe is stalled due to missing XON indication towards the NIG.

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>