rust: miscdevice: update ARef import to sync::aref

rust: miscdevice: update ARef import to sync::aref

Update the import of `ARef` in misc_device sample to use
`sync::aref` instead of `types`.

This is part of the ongoing effort to move `ARef` and
`AlwaysRefCounted` to the `sync` module for better modularity
and type organization.

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/20250818132806.134248-1-shankari.ak0208@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>