usb: gadget: Rename audio function to uac1

The extant USB-Audio function driver complies to UAC_1 spec.
So name the files accordingly, paving way for inclusion of
a new UAC_2 specified 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>