[S390] fix broken proc interface for sclp_async

This patch now allows the use of the proc interface to either activate
or deactivate call home on panic.
e.g.  echo 1 > /proc/sys/kernel/callhome
strict_strtoul() requires _either_'\n\0' _or_ '\0' termination.
This was missing and therefore the interface did not recognise valid input.

Signed-off-by: Hans-Joachim Picht <hans@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>