sound: Rename pcm_getparentchannel() to vchan_getparentchannel()

Follow the rest of the vchan.c naming convention.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45016

(cherry picked from commit 77ab4263bc42a5dcc102bbea39ce4f7f46b8d4f8)