staging: panel: fix error path

panel_attach() poorly handles errors. On error unregister everything we
have registered.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>