The intercept queue submit needs to be obstruction free as it can be
invoked by the runtime async handler helper thread. The code had a busy
wait loop waiting for a free slot to be available to add the retry
barrier packet. Blocking that thread prevents it servicing other async
handlers which may need to execute in order to allow packets on the
hardware queue to be processed to free up a slot.
Change the code to always leave one free slot unless there is a retry
barrier packet already on the queue.
Change-Id: If901c865550258b790b995d58037b0f99f1968cc
Tony Tye
d16c392338