[AE/osxsink] - fix possible device changed loop when using passthrough

Damnit - that one slipped in when i added the registration to the default output device changed callback.

@jmarshalnz for review - Gotham material sadly.
I guess you need to know when the format is changed from passthrough <-> non-passthrough, and if that is the case, unregister?

It seems that this is a workaround to the real fix though - what is the order of the callbacks being fired, and how are we responding to them that is causing the problem?