rust: auxiliary: consider auxiliary devices always have a parent

rust: auxiliary: consider auxiliary devices always have a parent

An auxiliary device is guaranteed to always have a parent device (both
in C and Rust), hence don't return an Option<&auxiliary::Device> in
auxiliary::Device::parent().

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>