usb: gadget: audio: open/close Audio files on USB connect/disconnect

Driver opens audio files on bind_config, but doesn't close them.
It prevents system to enter suspend state as AFE keeps pushing
data to apps. Change this design to open Audio streams on USB connect
(after set_alt) and close on disconnect i.e. function_disable. Also,
free usb_req and req->buff on disconnect after calling ep_disable as
buffers are currently not freed at all.

Change-Id: Ia6960c399aec2e082d04ad4409fa36880e87a5b7
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>