rust: pwm: Update ARef and AlwaysRefCounted imports to use sync::aref

rust: pwm: Update ARef and AlwaysRefCounted imports to use sync::aref

Update call sites in `pwm.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>
Acked-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://patch.msgid.link/20251123092438.182251-7-shankari.ak0208@gmail.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>