rust: device: conditionally expect dead_code for parent()

When `CONFIG_AUXILIARY_BUS` is disabled, `parent()` is still dead code:

    error: method `parent` is never used
      --> rust/kernel/device.rs:71:19
       |
    64 | impl<Ctx: DeviceContext> Device<Ctx> {
       | ------------------------------------ method in this implementation
    ...
    71 |     pub(crate) fn parent(&self) -> Option<&Self> {
       |                   ^^^^^^
       |
       = note: `-D dead-code` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(dead_code)]`

Thus reintroduce the `expect`, but now as a conditional one. Do so as
`dead_code` since that is narrower.

An `allow` would also be possible, but Danilo wants to catch new users
in the future [1].

Link: https://lore.kernel.org/rust-for-linux/aBE8qQrpXOfru_K3@pollux/ [1]
Fixes: ce735e7 ("rust: auxiliary: add auxiliary device / driver abstractions")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250429210629.513521-1-ojeda@kernel.org
[ Adjust commit subject to "rust: device: conditionally expect
  `dead_code` for `parent()`". - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: ojeda

rust/kernel/device.rs

@@ -68,6 +68,7 @@ impl<Ctx: DeviceContext> Device<Ctx> {
     }
 
     /// Returns a reference to the parent device, if any.
+    #[cfg_attr(not(CONFIG_AUXILIARY_BUS), expect(dead_code))]
     pub(crate) fn parent(&self) -> Option<&Self> {
         // SAFETY:
         // - By the type invariant `self.as_raw()` is always valid.