samd: audio_dma, audio_background: Gate with CIRCUITPY_ defines

Some ports which actually don't have audioio or audiobusio were still
calling into audio_dma_background().  This wasn't an error until
the assignment to audio_dma_state in audio_dma_stop was added, though
it's not clear why.

ports/atmel-samd/audio_dma.c

@@ -35,6 +35,8 @@
 #include "py/mpstate.h"
 #include "py/runtime.h"
 
+#if CIRCUITPY_AUDIOIO || CIRCUITPY_AUDIOBUSIO
+
 static audio_dma_t* audio_dma_state[AUDIO_DMA_CHANNEL_COUNT];
 
 // This cannot be in audio_dma_state because it's volatile.
@@ -348,3 +350,4 @@ void audio_dma_background(void) {
         audio_dma_pending[i] = false;
     }
 }
+#endif

ports/atmel-samd/background.c

@@ -56,7 +56,7 @@ void run_background_tasks(void) {
     assert_heap_ok();
     running_background_tasks = true;
 
-    #if (defined(SAMD21) && defined(PIN_PA02)) || defined(SAMD51)
+    #if CIRCUITPY_AUDIOIO || CIRCUITPY_AUDIOBUSIO
     audio_dma_background();
     #endif
     #if CIRCUITPY_DISPLAYIO