init: usb: fix audio_source function typo

for audio_source function instance, there exist 2 function name here:
    "audio_source.gs2" and "audio_source.gs3"
I believe this usb accessory audio name "audio_source.gs2" is a typo,
as there is no need to create another audio_source instance.

Test: Manual

Change-Id: I70b513db474632eb990131c285c8d5105c17970f
Signed-off-by: Winter Wang <wente.wang@nxp.com>