snd_uaudio: prefix module declaration with "snd_"

Although the module is compiled "snd_uaudio.ko", follow the rest of the
sound modules' naming convention in the declaration as well.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D43396

(cherry picked from commit 06a4376346e55d1c7aefe7a48063dddc4d1da9b9)