snd_uaudio(4): Fix config detection with defaults set.

Let the USB audio descriptor iteration detect configurations with more
channels and larger sample size, even when the following global sysctl
tunables are set to a lower value:

hw.usb.uaudio.default_channels hw.usb.uaudio.default_bits

This improves utility and is closer to the meaning of default.

Also, do not create duplicate sample rate entries.

MFC after: 2 weeks
Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D43679