s390/cpumf: remove superfluous nr_cpumask_bits check

Paul Burton reported that the nr_cpumask_bits check
within cpumsf_pmu_event_init() is not necessary.

Actually there is already a prior check within
perf_event_alloc(). Therefore remove the check.

Reported-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>