usb: gadget: audio: Move string IDs to audio.c

Move manufacturer and product string ids into audio.c so
as to be reusable by the new uac2 version of gadget driver.

Signed-off-by: Yadi Brar <yadi.brar01@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>