rust: pci: fix unrestricted &mut pci::Device

rust: pci: fix unrestricted &mut pci::Device

As by now, pci::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 pci::Device as

	pub struct Device<Ctx: DeviceContext = Normal>(
		Opaque<bindings::pci_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 pci::Device<Core>. Consequently, we
make this type accessible in bus callbacks only.

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

Fixes: 1bd8b6b2c5d3 ("rust: pci: add basic PCI device / driver abstractions")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250314160932.100165-4-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>