ANDROID: usb: f_accessory: Cancel any pending work before teardown

Tearing down and freeing the 'acc_dev' structure when there is
potentially asynchronous work queued involving its member fields is
likely to lead to use-after-free issues.

Cancel any pending work before freeing the structure.

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I68a91274aea18034637b738d558d043ac74fadf4
Signed-off-by: Giuliano Procida <gprocida@google.com>