s390/cpum_cf: rename per-CPU counter facility structure and variables

Rename the struct cpu_hw_events to cpu_cf_events and also the respective
per-CPU variable to make its name more clear.  No functional changes.

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