do GETBLKSIZE after setting bitrate, sample size, and channels

b/c the optimal fragment size depends on these settings, but also
GEBLKSIZE causes the fragment size to be set (which cannot be changed
w/o closing and reopening the device).
This fixes a problem a user was having with oss emu10k1 driver where
libao app's were skipping.


git-svn-id: http://svn.xiph.org/trunk/ao@5919 0101bb08-14d6-0310-b084-bc0e0c8e3800