core: lower maximum intensity to 31.

Intensity determines the number of threads started, is an unsigned int and
is used as (1 << intensity), so anything over 31 is meaningless.

miner.h

@@ -1047,8 +1047,8 @@ extern bool add_pool_details(struct pool *pool, bool live, char *url, char *user
 
 #define MIN_INTENSITY 8
 #define MIN_INTENSITY_STR "8"
-#define MAX_INTENSITY 42
+#define MAX_INTENSITY 31
-#define MAX_INTENSITY_STR "42"
+#define MAX_INTENSITY_STR "31"
 
 extern bool hotplug_mode;
 extern int hotplug_time;