cpuid: use unsigned for max cpuid

__get_cpuid_max returns an unsigned value.
For consistency, store the result in an unsigned variable.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

util/bufferiszero.c

@@ -272,7 +272,7 @@ static void init_accel(unsigned cache)
 
 static void __attribute__((constructor)) init_cpuid_cache(void)
 {
-    int max = __get_cpuid_max(0, NULL);
+    unsigned max = __get_cpuid_max(0, NULL);
     int a, b, c, d;
     unsigned cache = 0;