s390/cpum_sf: do not register PMU if no sampling mode is authorized

Previously, the cpum_sf PMU was registered even if there is no
sampling mode authorized.  Add a check and register cpum_sf only
at least one sampling mode is authorized.

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