Don't use the synchronization endpoint unless referenced by the
isochronous endpoint descriptor used for the data transfers, hence the
synchronization feature might not be supposed to be supported [yet].
This makes seamless playback synced with the USB HOST clock work with
the DN32-USB module for Midas audio systems and possibly other similar
products from Klark Teknik.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274918 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f