sound: Rename oss_audioinfo->real_device to oss_audioinfo->legacy_device

The OSS manual now documents this field as "legacy_device".

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45138

(cherry picked from commit 24d5cc14dd60b49553ff3ee7830305621f782ff1)