rust: auxiliary: add Driver::unbind() callback

Add missing unbind() callback to auxiliary::Driver, since it will be
needed by drivers eventually (e.g. the Nova DRM driver).

Acked-by: Alice Ryhl <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

rust/kernel/auxiliary.rs

···
         // SAFETY: `remove_callback` is only ever called after a successful call to
         // `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
         // and stored a `Pin<KBox<T>>`.
-        drop(unsafe { adev.as_ref().drvdata_obtain::<T>() });
+        let data = unsafe { adev.as_ref().drvdata_obtain::<T>() };
+
+        T::unbind(adev, data.as_ref());
     }
 }
 
···
     ///
     /// Called when an auxiliary device is matches a corresponding driver.
     fn probe(dev: &Device<device::Core>, id_info: &Self::IdInfo) -> impl PinInit<Self, Error>;
+
+    /// Auxiliary driver unbind.
+    ///
+    /// Called when a [`Device`] is unbound from its bound [`Driver`]. Implementing this callback
+    /// is optional.
+    ///
+    /// This callback serves as a place for drivers to perform teardown operations that require a
+    /// `&Device<Core>` or `&Device<Bound>` reference. For instance, drivers may try to perform I/O
+    /// operations to gracefully tear down the device.
+    ///
+    /// Otherwise, release operations for driver resources should be performed in `Self::drop`.
+    fn unbind(dev: &Device<device::Core>, this: Pin<&Self>) {
+        let _ = (dev, this);
+    }
 }
 
 /// The auxiliary device representation.