Add deprecation notice to snd_ds1(4).

As suggested in:
https://wiki.freebsd.org/WhatsGoing/FreeBSD13

We will be dropping the snd_ds1 driver. The driver is known to be buggy
and no one has been working on it for years now.
Users of old Yamaha cards may have luck with the OSS drivers instead.

MFC after: 3 days
Diferential Revision: https://reviews.freebsd.org/D21138