gpu: nova-core: Update ARef imports from sync::aref

gpu: nova-core: Update ARef imports from sync::aref

Update call sites in nova-core to import `ARef`
from `sync::aref` instead of `types`.

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

[acourbot@nvidia.com: use standard prefix for nova-core.]

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>
Link: https://lore.kernel.org/r/20250820112846.9665-1-shankari.ak0208@gmail.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>