uaudio patch for accepts only fixed sampling rate

[takawata at jp.freebsd.org]

Hi, I bought a USB speaker that accepts 
48khz audio bitrate only. And current uaudio code 
try to set if device cannot accept bitrate change request.

So I wrote a patch

http://www.init-main.com/uaudio.diff

This code is partly copy-n-paste from NetBSD specific
part now. With this code, upper layer can see it is 
not acceptable and invoke format converter.