bnx2x: Calling tx disable unconditionally

On unload, the FW assumes that no packets will be sent after the driver sends
the FW stop command. To ensure that, the driver must always call
netif_tx_disable

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