sierra: fix warnings with -Wswitch-enum

sierra/mm-broadband-modem-sierra.c: In function ‘modem_time_load_network_time’:
  sierra/mm-broadband-modem-sierra.c:1733:5: error: enumeration value ‘TIME_METHOD_UNKNOWN’ not handled in switch [-Werror=switch-enum]
   1733 |     switch (MM_BROADBAND_MODEM_SIERRA (self)->priv->time_method) {
        |     ^~~~~~

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

@@ -1737,6 +1737,7 @@ modem_time_load_network_time (MMIfaceModemTime *self,
     case TIME_METHOD_SYSTIME:
         command = "!SYSTIME?";
         break;
+    case TIME_METHOD_UNKNOWN:
     default:
         g_assert_not_reached ();
     }