cdcacm: fix memory leak in unbind

Freeing write requests seemingly worked, but since IN endpoint was freed
before write requests, 'priv->epbulkin' was NULL and cdcacm_freereq for
write requests as result was NOP.

Change order of freeing and add debug assert to cdcacm_freereq to make
such memory leaks more noticeable.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@haltian.com>
(cherry picked from commit 215b7fb129621d6fd7b7de40a34d737e69cecf8a)