sound: Make device registration more intuitive

The way a sound driver currently registers to sound(4) is using the
following sequence of function calls:

1. pcm_register() to initialize snddev_info.
2. pcm_addchan() calls to create the device's primary channels.
3. pcm_setstatus() to do the final setup.

While using 3 different functions in a specific order might not be very
elegant, this pattern cannot be easily avoided. However, pcm_register()
and pcm_setstatus() are especially confusing, since one would
intuitively expect:

1. pcm_register() to actually do the registration, as opposed to a basic initialization.
2. pcm_setstatus() to, as the name suggests, set some kind of status, as opposed to finalizing the registration.

This patch renames pcm_register() to pcm_init(), and pcm_setstatus() to
pcm_register(). Drivers are modified accordingly.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D47325