sound: Move root feeder initialization to separate function

feeder_register() is currently a SYSINIT in order to create the root
feeder, which happens only when feedercnt is 0. Separating the root
feeder registration makes the code more readable.

No functional change intended.

While here, fix some style errors.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, zlei, markj
Differential Revision: https://reviews.freebsd.org/D46821