usb: gadget: f_uac2: use defined constants as defaults

When configfs is integrated the same values will have to be used
as defaults. Use symbolic names in order not to duplicate magic numbers.

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>