rtio: Properly track last sqe in the queue

The pending_sqe logic to track where in the ring queue the concurrent
executor had left off was slightly flawed. It didn't account for starting
all sqes in the queue and ending back up at the beginning.

Instead track the last SQE in the queue, from which the next one in the
queue will the one to start next.

If we happen to sweep the last known SQE in the queue, reset it to NULL
so the next time prepare is called we start at the beginning of the queue
again.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>