bnx2x: use smp_mb() to keep ordering of read write operations

Since we want to keep ordering of write to fp->bd_tx_cons and
netif_tx_queue_stopped(txq), what is read of txq->state, we have to use
general memory barrier.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>