bnx2x: change smp_mb() comment to conform the true

Access to fp->tx_bp_prod is protected by __netif_tx_lock,
smp_mb() is not needed for that.

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