rust: auxiliary: unregister on parent device unbind

rust: auxiliary: unregister on parent device unbind

Guarantee that an auxiliary driver will be unbound before its parent is
unbound; there is no point in operating an auxiliary device whose parent
has been unbound.

In practice, this guarantee allows us to assume that for a bound
auxiliary device, also the parent device is bound.

This is useful when an auxiliary driver calls into its parent, since it
allows the parent to directly access device resources and its device
private data due to the guaranteed bound device context.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>