sound: Add AFMT_FLOAT to feed_volume_info_tab

Fixes: e1bbaa71d62c ("sound: Implement AFMT_FLOAT support")
Reported by: bz
Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D50793

(cherry picked from commit 4a7b8cd7057f0c37b4fc4b47b0ef3ec8402b0eb9)