IB/hfi1: Correct defered count after processing qp_wait_list

The qp_wait_list processing leaves the defered ack count
at its prior value.

This can result in a premature send of an ack.

Fixed by unconditionally reseting the defered ack count
in hfi1_send_rc_ack().

Fixes: Commit 7c091e5c ("staging/rdma/hfi1: add ACK coalescing logic")
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>