port-serial-at: make mm_port_serial_at_command_finish() transfer ownership

This will make it slightly easier to port mm_port_serial_at_command() to
use GTask, since we won't have to keep a pointer to the result in GTask
after _finish() has been called.

In most cases the caller needs the value anyway, so this doesn't add too
much hasle.

src/plugins/dell/mm-plugin-dell.c

@@ -158,7 +158,7 @@ response_ready (MMPortSerialAt *port,
 {
     CustomInitContext *ctx;
     MMPortProbe       *probe;
-    const gchar       *response;
+    g_autofree gchar  *response = NULL;
     GError            *error = NULL;
     gchar             *lower;
     DellManufacturer   manufacturer;