s390/perf: Change CPUM_CF return code in event init function

The function perf_init_event() creates a new event and
assignes it to a PMU. This a done in a loop over all existing
PMUs. For each listed PMU the event init function is called
and if this function does return any other error than -ENOENT,
the loop is terminated the creation of the event fails.

If the event is invalid, return -ENOENT to try other PMUs.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>