qed: Add ll2 option for dropping a tx packet

The option of sending a packet on the ll2 and dropping it exists in
hardware and was not used until now, thus not exposed.
The iWARP unaligned MPA flow requires this functionality for
flushing the tx queue.

Signed-off-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Signed-off-by: Ariel Elior <Ariel.Elior@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>