sound: Add missing oss_mixerinfo devnode and legacy_device fields

They are missing from soundcard.h and are in fact used by some
applications, such as OSS' ossinfo(1):
http://manuals.opensound.com/developer/ossinfo.c.html

The new size for filler is chosen according to the most recent official
version of soundcard.h, which includes those 2 fields.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45137