iodevices.LWP3Device: return bytes written

This changes the return value of the LWP3Device.write() method to return
the number of bytes written. This makes it compatible with other Python
write() functions and reserves returning None for a possible future
non-blocking write.

pybricks/iodevices/pb_type_iodevices_lwp3device.c

@@ -176,7 +176,7 @@ STATIC mp_obj_t lwp3device_write(mp_obj_t self_in, mp_obj_t buf_in) {
     pbdrv_bluetooth_write_remote(&lwp3device->task, &msg.value);
     pb_wait_task(&lwp3device->task, -1);
 
-    return mp_const_none;
+    return MP_OBJ_NEW_SMALL_INT(bufinfo.len);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(lwp3device_write_obj, lwp3device_write);