cxgb4: add Tx and Rx path for ETHOFLD traffic

Implement Tx path for traffic flowing through software EOSW_TXQ
and EOHW_TXQ. Since multiple EOSW_TXQ can post packets to a single
EOHW_TXQ, protect the hardware queue with necessary spinlock. Also,
move common code used to generate TSO work request to a common
function.

Implement Rx path to handle Tx completions for successfully
transmitted packets.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>