sound: Export hardware and software buffer sample rate in sndstat nvlist

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49983

(cherry picked from commit 19ec522d6dc1d4616e07f5668339fc637856f547)

sound tests: Fetch HWBUF_RATE and SWBUF_RATE in sndstat_nv

Introduced in 19ec522d6dc1 ("sound: Export hardware and software buffer
sample rate in sndstat nvlist").

Sponsored by: The FreeBSD Foundation
MFC after: 1 week

(cherry picked from commit 52a0f967dcc5b78c5f2f7e49a96e9cec56472a4f)