rust: opp: update ARef and AlwaysRefCounted imports from sync::aref

rust: opp: update ARef and AlwaysRefCounted imports from sync::aref

Update call sites in `opp.rs` to import ARef and
AlwaysRefCounted from sync::aref instead of types.

This aligns with the ongoing effort to move ARef and
AlwaysRefCounted to sync.

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1173
Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>