sound: Make SNDST_UNVLBUF_MAX a power of two

Fixes: 074d337ad618 ("sound: Check user-supplied size passed to SNDSTIOC_ADD_USER_DEVS*")
Reported by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45277

(cherry picked from commit d59058f3b31353442497e728dd5beb80585ad8d2)
(cherry picked from commit eba949c37e6473a07a6e3a386f239aa0f49a4c93)

Approved by: re (cperciva)