usb: gadget: Kconfig: enable separate compilation of uac1/uac2 functions

uac1 and uac2 functions are available through the configfs interface
and it should be possible to build them without building their legacy
gadgets.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>