usb: gadget: f_uac1: use defined constants as defaults

When configfs support is added the values in question will have to be
used in two different places. Substitute them with defined constants
to avoid duplicating 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>