helpers: new macro to CLAMP high threshold only

Useful when clamping a unsigned integer with low threshold set to 0,
which would give us compiler warnings with -Wtype-limits when using
CLAMP(), e.g.:

  via/mm-broadband-modem-via.c: In function ‘handle_evdo_quality_change’:
  /usr/include/glib-2.0/glib/gmacros.h:811:63: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
    811 | #define CLAMP(x, low, high)  (((x) > (high)) ? (high) : (((x) < (low)) ? (low) : (x)))
        |                                                               ^
  via/mm-broadband-modem-via.c:284:19: note: in expansion of macro ‘CLAMP’
    284 |         quality = CLAMP (quality, 0, 100);
        |                   ^~~~~

plugins/option/mm-broadband-modem-option.c

@@ -808,7 +808,7 @@ option_signal_changed (MMPortSerialAt *port,
                        MMBroadbandModemOption *self)
 {
     gchar *str;
-    gint quality = 0;
+    guint quality = 0;
 
     str = g_match_info_fetch (match_info, 1);
     if (str) {
@@ -821,10 +821,10 @@ option_signal_changed (MMPortSerialAt *port,
         quality = 0;
     } else {
         /* Normalize the quality */
-        quality = CLAMP (quality, 0, 31) * 100 / 31;
+        quality = MM_CLAMP_HIGH (quality, 31) * 100 / 31;
     }
 
-    mm_iface_modem_update_signal_quality (MM_IFACE_MODEM (self), (guint)quality);
+    mm_iface_modem_update_signal_quality (MM_IFACE_MODEM (self), quality);
 }
 
 static void