stm32: Refactor DMA interrupts

Previously, every dma interrupt handler called the same `on_irq`
function which had to check the state of every dma channel.

Now, each dma interrupt handler only calls an `on_irq` method for its
corresponding channel or channels.

embassy-stm32/src/dma/mod.rs

@@ -88,6 +88,11 @@ pub(crate) mod sealed {
 
         /// Sets the waker that is called when this channel stops (either completed or manually stopped)
         fn set_waker(&mut self, waker: &Waker);
+
+        /// This is called when this channel triggers an interrupt.
+        /// Note: Because some channels share an interrupt, this function might be
+        /// called for a channel that didn't trigger an interrupt.
+        fn on_irq();
     }
 }