cxgb4: Free completed tx skbs promptly

Description of problem:
The NIC card is not reporting back to the driver the transmitted skbs,
so they get stuck in the TX ring causing issues with reference
counters in other kernel components.

Developed a new Automatic Egress Queue Update firmware facility to slowly tick
through Egress Queues and send back any outstanding CIDX Updates which are
laying around.

Based on original work by Casey Leedom <leedom@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>