[Bugfix] FS#13492 - Playback resume fails after USB insert
On usb plug system_flush() gets called before the other
threads get a chance to ACK the insert
system_flush() calls playlist_shutdown() (amongst other things)
playlist_shutdown closes the control file
the audio thread acks USB insert
audio_stop_playback is called which now can't look up the
filename from the now closed handle for control the control file
I left the guard for checking for USB insert but it appears to be
fine without it
Change-Id: I935dbf7aed38d4a57413c0063ad953f427e9b3bb