sound: Retire volch

volch is a variable used internally in dsp_ioctl_channel() which
corresponds to either priv->rdch or priv->wrch. As a result, priv->volch
is also a pointer to one of those two channels, but is also not used
anywhere outside dsp_ioctl_channel(). Since the operation of figuring
out which channel we want to operate on inside dsp_ioctl_channel() is
cheap enough, retire volch to avoid unnecessary complexity and
confusion.

While here, rename "volch" to just "ch".

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50022