s390/qdio: refactor ACK processing for primed SBALs
inbound_primed() currently has two code paths - one for QEBSM that knows
how to deal with multiple ACKs, and a non-QEBSM path that strictly
assumes a single ACK on the queue.
In preparation for a subsequent patch, slightly adjust the non-QEBSM
path so that it can manage a queue with multiple ACKs.