How do I attach this to a driver? Do I fill in a ac97_host_if, then call
ac97_attach?

Btw: 

The hardware contains a FIFO per direction for the data, interupt logic for
the FIFO, a switch to connect the FIFO either to the AC97 or to a custom
S/PDIF interface (but that is 32/44.1/48/96k, 24/32bit only), while the
codec builtin s/pdif output can accept data at all ac97 data formats and
convert it, I think.)

The codec is currently (but not guaranteed on all board revision) a 
cs4299.

	-is