sound: Simplify feeder_getclass()

With the parameters we provide feeder_getclass(), the only thing we
really care about is the feeder's class type. We can simplify (and make
the code more readable) that by simply passing the type to
feeder_getclass(). Apart from being simpler, we can now also retire
feedertab_entry->desc and cmpdesc().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53553