RDMA/cxgb3: Connection termination fixes

The poll and flush code needs to handle all send opcodes: SEND,
SEND_WITH_SE, SEND_WITH_INV, and SEND_WITH_SE_INV.

Ignore TERM indications if the connection already gone.

Ignore HW receive completions if the RQ is empty.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>