usb: gadget: audio: Use container_of to free audio_dev

Eliminate static struct *agdev_g from f_uac2.c.
It is used for freeing its memory, but the same address can be found
by calling container_of in afunc_unbind(). This implies eliminating
uac2_unbind_config(). The audio_config_driver in audio.c does not have
its unbind method any more. It has been used only when uac2 is used,
so uac2 itself can handle unbinding in afunc_unbind().

Tested-by: Sebastian Reimers <sebastian.reimers@googlemail.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>