s390/pai: rename structure member users to active_events

Rename structure member users to active_events to make it consistent
with PMU pai_ext. Also use the same prefix syntax for increment and
decrement operators in both PMUs.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>