s390/cpum_cf: make hw_perf_event_update() a void function

The return code of hw_perf_event_update() is not evaluated by
its callers.  Hence, simplify the function by removing the
return code.

Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>