rust: platform: fix unrestricted &mut platform::Device

As by now, platform::Device is implemented as:

	#[derive(Clone)]
	pub struct Device(ARef<device::Device>);

This may be convenient, but has the implication that drivers can call
device methods that require a mutable reference concurrently at any
point of time.

Instead define platform::Device as

	pub struct Device<Ctx: DeviceContext = Normal>(
		Opaque<bindings::platform_dev>,
		PhantomData<Ctx>,
	);

and manually implement the AlwaysRefCounted trait.

With this we can implement methods that should only be called from
bus callbacks (such as probe()) for platform::Device<Core>. Consequently,
we make this type accessible in bus callbacks only.

Arbitrary references taken by the driver are still of type
ARef<platform::Device> and hence don't provide access to methods that are
reserved for bus callbacks.

Fixes: 683a63befc73 ("rust: platform: add basic platform device / driver abstractions")
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Danilo Krummrich <[email protected]>
Acked-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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