s390/perf: cpum_cf: fallback to software sampling events

The CPU-measurement counter facility does not support sampling events
and returns -EINVAL in that case.  This return code lets the perf tool
fail.  To fall back to software sampling events, return -ENOENT instead.

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