ANDROID: usb: f_accessory: Fix teardown ordering in acc_release()

acc_release() attempts to synchronise with acc_open() using an atomic
'open_excl' member in 'struct acc_dev'. Unfortunately, acc_release()
prematurely resets this atomic variable to zero, meaning there is a
potential race on 'dev->disconnected':

acc_open()				acc_release()
						atomic_xchg(open_excl), 0)
	atomic_xchg(open_excl, 1)
	dev->disconnected = 0;			dev->disconnected = 1;

Fix the race by ensuring that the 'disconnected' field is written
before clearing 'open_excl' in acc_release().

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Ib9a21f2305f6d70de3e760da62dbfdd66889200a
Signed-off-by: Giuliano Procida <gprocida@google.com>