bnx2x: Prevent restarting Tx during bnx2x_nic_unload

Tx queues were stopped before  bp->state was changed to a value different 
from BNX2X_STATE_OPEN, which allowed the bnx2x_tx_int() called from the 
NAPI context to re-enable it. This then allowed the netdev->ndo_start_xmit() 
to be called in the middle of the function reset and rings freeing. 

This patch changes bp->state to a value different 
from BNX2X_STATE_OPEN BEFORE disabling the Tx queues in order to restore the
broken protection against the above race in the bnx2x_tx_int().

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