usb: gadget: audio: Don't attempt to close already closed files

Audio function driver (f_uac1.c) may call gaudio_cleanup back
to back without calling gaudio_setup. Handle this gracefully
by marking file pointer as NULL on file_close as gaudio_cleanup
routine already checks for NULL before file_close.

Change-Id: Ib6acb05578105d64cceaa766d0b0434d1a467c8f
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>