ANDROID: usb: f_accessory: Don't corrupt global state on double registration

If acc_setup() is called when there is already an allocated instance,
misc_register() will fail but the error path leaves a dangling pointer
to freed memory in the global 'acc_dev' state.

Fix this by ensuring that the refcount is zero before we start, and then
using a cmpxchg() from NULL to serialise any concurrent initialisers.

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I2c26289dcce7dbc493964516c49b05d04aaa6839
Signed-off-by: Giuliano Procida <gprocida@google.com>