USB: f_audio_source: Increase the IN endpoint buffer allocation

Some UDC may require allocation of some extra bytes for TX buffer
due to hardware requirement. Add necessary changes for the same.

Change-Id: I3cedcb6e63d2c8c08d17717fa0d8ce692381b1ea
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>