USB: sierra: fix memory leak in attach error path

commit 7e41f9bcdd2e813ea2a3c40db291d87ea06b559f upstream.

Make sure port private data is deallocated on errors in attach.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>