USB: f_audio_source: Use different names for multi instances

Currently userspace creates two directories due to which
multi instances of audiosource dev created. But while creating
device, same name is being used due to which 2nd instance
allocation fails. Fix this by using different names for multi
instants of audio source device as expected by android framework.

Change-Id: Icabf6bf21cdf3f7d94eafda81fe51f3f95c2bbf4
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>