bnx2x: stop tx before CNIC_STOP

It may take some time to cnic to respond, this prevents tx_timeout
when it happens.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>