s390/cpum_cf: cleanup event/counter validation

The validate_event() function just checked for reserved counters
in particular CPU-MF counter sets.  Because the number of counters
in counter sets vary among different hardware models, remove the
explicit check to tolerate new models.

Reserved counters are not accounted and, thus, will return zero.

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