broadband-modem: update signal quality normalization

This patch updates normalize_ciev_cind_signal_quality() in
MMBroadbandModem to remove an unnecessary check on 'quality >= 0' and
also makes sure the normalized signal quality is capped at 100 when no
maximum is specified.

This is revised from a patch originally authored by
Yunlian Jiang <yunlian@chromium.org>.

Bug reported on https://code.google.com/p/chromium/issues/detail?id=235989

src/mm-broadband-modem.c

@@ -1651,11 +1651,10 @@ normalize_ciev_cind_signal_quality (guint quality,
                                     guint min,
                                     guint max)
 {
-    if (!max &&
-        quality >= 0) {
+    if (!max) {
         /* If we didn't get a max, assume it was 5. Note that we do allow
          * 0, meaning no signal at all. */
-        return (quality * 20);
+        return (quality <= 5) ? (quality * 20) : 100;
     }
 
     if (quality >= min &&