s390/qdio: simplify ACK tracking

Current code uses a 'polling' flag to keep track of whether an Input
Queue has any ACKed SBALs. QEBSM devices might have multiple ACKed
SBALs, and those are tracked separately with 'ack_count'.

By also setting ack_count for non-QEBSM devices (to a fixed value of 1),
we can use 'ack_count != 0' as replacement for the polling flag.

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