sound: Handle multiple primary channel cases in vchan sysctls

vchan_getparentchannel() is used by various vchan sysctl functions to
fetch the first primary channel. However, this assumes that all devices
have only one primary channel per direction. If a device does not meet
this assumption, then the sysctl functions will be applying the
configurations on the first primary channel only.

Since we now have the "primary" channel sublist, we can retire
vchan_getparentchannel() and iterate through the "primary" list in each
sysctl function and apply the settings to all primary channels.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48336