snd_uaudio: mark selected configurations

snd_uaudio(4) selects the first maching rate/channel/bit/format/buffer
configuration for use during attach, even though it will print the rest
of the supported configurations detected. To make this clear, mark the
selected playback and recording configurations with a "selected" string.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D43766