[S390] call home: fix local buffer usage in proc handler

Fix the size of the local buffer and use snprintf to prevent
further miscalculations. Also fix the usage of bitwise vs logic
operations.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>