usb: gadget: f_uac1: prepare for separate compilation

Integrating configfs requires converting f_uac1 to new function interface,
which in turn requires converting it to the new function interface,
which involves separate compilation of f_uac1.c into usb_f_uac1.ko.
u_uac1.c contains some module parameters. After this patch is applied
they are still a part of the resulting g_audio.ko, but can be guarded
with a compatiblity flag which will be removed when no users of the old
function interface of f_uac1 are left.

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>