cxgb4: fix EOTID leak when disabling TC-MQPRIO offload

Under heavy load, the EOTID termination FLOWC request fails to get
enqueued to the end of the Tx ring due to lack of credits. This
results in EOTID leak.

When disabling TC-MQPRIO offload, the link is already brought down
to cleanup EOTIDs. So, flush any pending enqueued skbs that can't be
sent outside the wire, to make room for FLOWC request. Also, move the
FLOWC descriptor consumption logic closer to when the FLOWC request is
actually posted to hardware.

Fixes: 0e395b3c ("cxgb4: add FLOWC based QoS offload")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>