staging/rdma/hfi1: use mod_timer when appropriate

Use new timer API to optimize maintenance of
timers during ACK processing.

When we are still expecting ACKs, mod the timer
to avoid a heavyweight delete/add. Otherwise, insure
do_rc_ack() maintains the timer as it had.

Reviewed-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>