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