IB/hfi1: Fix PIO wakeup timing hole

There is a timing hole if there had been greater than
PIO_WAIT_BATCH_SIZE waiters.  This code will dispatch the first
batch but leave the others in the queue.   If the restarted waiters
don't in turn wait on a buffer, there is a hang.

Fix by forcing a return when the QP queue is non-empty.

Reviewed-by: Vennila Megavannan <vennila.megavannan@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>