usb: gadget: f_uac1: add configfs support

Add support for using f_uac1 function as a component of a gadget
composed with configfs.

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>