snd_uaudio: Retire sndcard_func usage

This is effectively a no-op, as it does not make use of the
sndcard_func->varinfo field, so eventually ua_probe() always succeeds.

Also change ua_probe()'s value to 0. There is no need to return
BUS_PROBE_DEFAULT, because snd_uaudio() attaches the sound(4)'s children
with bus_attach_children().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D56160