shared-qmi: fix warnings with -Wswitch-enum

mm-shared-qmi.c: In function ‘pds_set_default_tracking_session_ready’:
  mm-shared-qmi.c:4209:5: error: enumeration value ‘GPS_OPERATION_MODE_UNKNOWN’ not handled in switch [-Werror=switch-enum]
   4209 |     switch (ctx->mode) {
        |     ^~~~~~
  mm-shared-qmi.c: In function ‘loc_location_set_operation_mode_indication_cb’:
  mm-shared-qmi.c:4354:5: error: enumeration value ‘GPS_OPERATION_MODE_UNKNOWN’ not handled in switch [-Werror=switch-enum]
   4354 |     switch (ctx->mode) {
        |     ^~~~~~

src/mm-shared-qmi.c

@@ -4216,6 +4216,7 @@ pds_set_default_tracking_session_ready (QmiClientPds *client,
         case GPS_OPERATION_MODE_STANDALONE:
             mm_dbg ("Standalone mode enabled (A-GPS disabled)");
             break;
+        case GPS_OPERATION_MODE_UNKNOWN:
         default:
             g_assert_not_reached ();
     }
@@ -4361,6 +4362,7 @@ loc_location_set_operation_mode_indication_cb (QmiClientLoc
         case GPS_OPERATION_MODE_STANDALONE:
             mm_dbg ("Standalone mode enabled (A-GPS disabled)");
             break;
+        case GPS_OPERATION_MODE_UNKNOWN:
         default:
             g_assert_not_reached ();
     }