Remove unnecessary use of atomic-polyfill.

Only use it when CAS is actually needed.

embassy-stm32/src/usart/buffered.rs

@@ -1,8 +1,8 @@
 use core::cell::RefCell;
 use core::future::poll_fn;
+use core::sync::atomic::{compiler_fence, Ordering};
 use core::task::Poll;
 
-use atomic_polyfill::{compiler_fence, Ordering};
 use embassy_cortex_m::peripheral::{PeripheralMutex, PeripheralState, StateStorage};
 use embassy_hal_common::ring_buffer::RingBuffer;
 use embassy_sync::waitqueue::WakerRegistration;