RDMA/cxgb4: Keep QP referenced until TID released

The driver is currently releasing the last ref on the QP too early.
This can cause bus errors due to HW still fetching WRs from the HW
queue.  The fix is to keep a qp ref until we release the HW TID.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>