s390/topology: let topology_mnest_limit() return unsigned char

Fixes a couple of compile warnings with gcc 7.1.0 :

arch/s390/kernel/sysinfo.c:578:20:
  note: directive argument in the range [-2147483648, 4]
   sprintf(link_to, "15_1_%d", topology_mnest_limit());

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>