rust: auxiliary: move parent() to impl Device

rust: auxiliary: move parent() to impl Device

Currently, the parent method is implemented for any Device<Ctx>, i.e.
any device context and returns a &device::Device<Normal>.

However, a subsequent patch will introduce

	impl Device<Bound> {
	    pub fn parent() -> device::Device<Bound> { ... }
	}

which takes advantage of the fact that if the auxiliary device is bound
the parent is guaranteed to be bound as well.

I.e. the behavior we want is that all device contexts that dereference
to Bound, will use the implementation above, whereas the old
implementation should only be implemented for Device<Normal>.

Hence, move the current implementation.

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