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.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>