sound: Simplify feeder_getclass()

[christos]

Summary

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

[markj]

sys/dev/sound/pcm/feeder.c

59

Why is it "first" if there's only one?

[christos]

sys/dev/sound/pcm/feeder.c

59

This function is called by the following sysinit:

SYSINIT(feeder_root, SI_SUB_DRIVERS, SI_ORDER_FIRST, feeder_register_root,
    &feeder_root_class);

This one is called before the other feeder sysinits. The check here is just to make sure that we enter this function for FEEDER_ROOT only.