broadband-modem: prefer explicit return to break

Just to make it explicit that there is no more logic running
afterwards.

src/mm-broadband-modem.c

@@ -12666,19 +12666,19 @@ enable (MMBaseModem         *self,
         g_task_return_new_error (task, MM_CORE_ERROR, MM_CORE_ERROR_WRONG_STATE,
                                  "Cannot enable modem: unknown state");
         g_object_unref (task);
-        break;
+        return;
 
     case MM_MODEM_STATE_FAILED:
         g_task_return_new_error (task, MM_CORE_ERROR, MM_CORE_ERROR_WRONG_STATE,
                                  "Cannot enable modem: initialization failed");
         g_object_unref (task);
-        break;
+        return;
 
     case MM_MODEM_STATE_LOCKED:
         g_task_return_new_error (task, MM_CORE_ERROR, MM_CORE_ERROR_WRONG_STATE,
                                  "Cannot enable modem: device locked");
         g_object_unref (task);
-        break;
+        return;
 
     case MM_MODEM_STATE_INITIALIZING:
     case MM_MODEM_STATE_DISABLED:
@@ -12688,7 +12688,7 @@ enable (MMBaseModem         *self,
 
     case MM_MODEM_STATE_ENABLING:
         g_assert_not_reached ();
-        break;
+        return;
 
     case MM_MODEM_STATE_ENABLED:
     case MM_MODEM_STATE_SEARCHING:
@@ -12699,7 +12699,7 @@ enable (MMBaseModem         *self,
         /* Just return success, don't relaunch enabling */
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
-        break;
+        return;
 
     default:
         g_assert_not_reached ();