s390/sclp_async: replace callhome proc handler with generic one

Reuse generic proc_dointvec_minmax proc handler instead of
proc_handler_callhome.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>