sound: Move global variable initialization to sound_modevent()

There is no reason to initialize global variables in feeder_register(),
as these variables are unrelated to what the function does. Instead,
initialize them during sound(4) load.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: zlei
Differential Revision: https://reviews.freebsd.org/D46749

(cherry picked from commit 0a0301deb5b6a9c66829dd20cff9d40c5ba9ad92)