USB: f_audio_source: Enable audio endpoint for alternate setting one

audio_source driver is having two interfaces and also audio streaming
interface supports alternate settings. Only alternate setting one has
endpoint. But other two interfaces have no endpoints. Hence endpoint is
required to enable for only alternate setting one. But currently driver
is enabling endpoint whenever set_alt callback is called. Due to this,
enable endpoint will be called multiple times which causes exception.
Fix this by enabling endpoint only for alternate setting one.

Change-Id: Ib5910bf663f250250e8355bbdd746116e890f6d0
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>