sound: Get rid of pnum and max variables in chn_init()

The VCHAN count is checked in vchan_setnew(), and there is no reason to
cap the hardware channels in the first place.

This is part of a series of follow-up patches.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46521

(cherry picked from commit de8ee333388bf5e551e697b08823b3a6c262f609)