usb: gadget: f_uac2: introduce agdev_to_uac2_opts

Add a simple container_of() wrapper to get a struct f_uac2_opts from a
struct struct audio_dev. Use it in two places where it is currently
open-coded.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>