codal_port/microbit_pinaudio: Disallow pin_speaker for audio pin select.

Audio/music/speech is output on the internal speaker by default, and can be
turned off via speaker.off().

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

src/codal_port/microbit_pinaudio.c

@@ -35,6 +35,8 @@ void microbit_pin_audio_select(mp_const_obj_t select) {
     const microbit_pin_obj_t *pin_selected;
     if (select == mp_const_none) {
         pin_selected = NULL;
+    } else if (select == MP_OBJ_FROM_PTR(&microbit_pin_speaker_obj)) {
+        mp_raise_ValueError("pin_speaker not allowed");
     } else {
         pin_selected = microbit_obj_get_pin((mp_obj_t)select);
     }