s390/cpum_cf: simplify pr_err() statement in cpumf_pmu_enable/disable

Simplify pr_err() statement into one line and omit return statement.

No functional change.

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>