rust: driver: address soundness issue in `RegistrationOps`

The `RegistrationOps` trait holds some obligations to the caller and
implementers. While being documented, the trait and the corresponding
functions haven't been marked as unsafe.

Hence, markt the trait and functions unsafe and add the corresponding
safety comments.

This patch does not include any fuctional changes.

Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://lore.kernel.org/rust-for-linux/20241224195821.3b43302b.gary@garyguo.net/


Signed-off-by: Danilo Krummrich <dakr@kernel.org>