usbd: Refactor MIDI to work with latest set of changes.

Drops the idea of having more than one TX or RX "cable", it's a nightmare
to debug this all the way through the OS MIDI Stack. Can still be added in
future if needed.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

Author: projectgus

micropython/usbd/examples/midi_example.py

@@ -0,0 +1,45 @@
+# MicroPython USB MIDI example
+# MIT license; Copyright (c) 2023 Angus Gratton
+import usbd
+import time
+
+
+class MIDIExample(usbd.MIDIUSB):
+    def on_note_on(self, channel, pitch, vel):
+        print(f"RX Note On channel {channel} pitch {pitch} velocity {vel}")
+
+    def on_note_off(self, channel, pitch, vel):
+        print(f"RX Note Off channel {channel} pitch {pitch} velocity {vel}")
+
+    def on_control_change(self, channel, controller, value):
+        print(f"RX Control channel {channel} controller {controller} value {value}")
+
+
+ud = usbd.get_usbdevice()
+
+m = MIDIExample()
+ud.add_interface(m)
+ud.reenumerate()
+
+print("Waiting for interface...")
+
+while not m.is_open():
+    time.sleep_ms(100)
+
+print("Starting MIDI loop...")
+
+control_val = 0
+channel = 0
+
+
+while m.is_open():
+    time.sleep(1)
+    m.note_on(channel, 60)
+    time.sleep(0.5)
+    m.note_off(channel, 60)
+    time.sleep(1)
+    m.control_change(channel, 64, control_val)
+    control_val += 1
+    if control_val == 0x7F:
+        control_val = 0
+    time.sleep(1)