IB/rdmavt: Increase CQ callback thread priority

The priority of the send engines is higher than the CQ completion
thread potentially causing completions to be starved for very
fast interfaces.

Change the CQ kthread to match the send engine threads to minimize
this delay for ULP completion processing.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>