cxgbe/iw_cxgbe: Implement sq/rq drain operation.

ULPs can set a qp's state to ERROR and then post a work request on the
sq and/or rq. When the reply for that work request comes back it is
guaranteed that all previous work requests posted on that queue have
been drained.

Obtained from: Chelsio Communications
MFC after: 3 days
Sponsored by: Chelsio Communications