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>

arch/s390/include/asm/sysinfo.h

@@ -146,7 +146,7 @@ extern int topology_max_mnest;
  * Returns the maximum nesting level supported by the cpu topology code.
  * The current maximum level is 4 which is the drawer level.
  */
-static inline int topology_mnest_limit(void)
+static inline unsigned char topology_mnest_limit(void)
 {
 	return min(topology_max_mnest, 4);
 }