bnx2x: tx_has_work should not wait for FW

The current tx_has_work waited until all packets sent by the driver
are marked as completed by the FW. This is too greedy and it causes
the bnx2x_poll to spin in vain. The driver should only check that all
packets FW already completed are freed - only in unload flow the
driver should make sure that transmit queue is empty

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>