samd: audio_dma_stop: Clear out audio_dma_state[]

As identified in #1908, when both AudioOut and PDMIn are used, hard
locks can occur.  Because audio_dma_stop didn't clear audio_dma_state[],
a future call to audio_dma_load_next_block could occur using a DMA
object which belongs to PDMIn.

I believe that this Closes: #1908 though perhaps it is still not the full
story.

Testing performed: Loaded a sketch similar to the one on #1908 that
tends to reproduce the bug within ~30s.  Ran for >300s without hard
lock.  HOWEVER, while my cpx is no longer hard locking, it occasionally
(<1 / 200s) announces
    Code done running. Waiting for reload.
(and does so), even though my main loop is surrounded by a 'while True:'
condition, so there are still gremlins nearby.

ports/atmel-samd/audio_dma.c

@@ -261,6 +261,7 @@ void audio_dma_stop(audio_dma_t* dma) {
     dma_disable_channel(dma->dma_channel);
     disable_event_channel(dma->event_channel);
     MP_STATE_PORT(playing_audio)[dma->dma_channel] = NULL;
+    audio_dma_state[dma->dma_channel] = NULL;
 
     dma->dma_channel = AUDIO_DMA_CHANNEL_COUNT;
 }