sound: Get rid of snd_sb16 workaround in vchan_create()

The snd_sb16 driver was removed in 716924cb4832 ("Retire snd_sbc ISA
sound card driver").

While here, simplify sample rate assignment a bit.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D45662