pcm: centralize 32-bit ioctl compat

Move all handlng of struct sndstioc_nv_arg(32) to sndstat_ioctl() and
make the functions that actually do the work take a buffer and size or
size pointer. The 32-bit compat work is minimal so just inline it.

Remove checks that we've got a 32-bit process for 32-bit ioctls. We
don't check that default ioctls are from 64-bit processes on 64-bit
systems.

Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D45307