port-serial: fix warnings with -Wsign-compare

mm-port-serial.c: In function ‘_close_internal’:
  mm-port-serial.c:1464:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘guint’ {aka ‘unsigned int’} [-Werror=sign-compare]
   1464 |     for (i = 0; i < g_queue_get_length (self->priv->queue); i++) {
        |                   ^

src/mm-port-serial.c

@@ -1372,7 +1372,7 @@ static void
 _close_internal (MMPortSerial *self, gboolean force)
 {
     const char *device;
-    int i;
+    guint       i;
 
     g_return_if_fail (MM_IS_PORT_SERIAL (self));