Bluetooth: Host: Cancel pending deferred work on disconnect

This fixes an issue that occurred if deferred_work is queued by another
procedure before we run bt_conn_set_state(BT_CONN_DISCONNECTED).
bt_conn_set_state yields to the system work queue in bt_conn_tx_notify,
and then deferred_work runs and handles the disconnected callbacks.
bt_conn_set_state then enqueues another deferred_work which calls the
disconnected callbacks again and causes an assert. k_work_cancel_delayable
will clear queued deferred work when we call
bt_conn_set_state(BT_CONN_DISCONNECTED), guaranteeing that the disconnect
callbacks will only be called once.

Signed-off-by: Timothy Keys <timothy.keys@nordicsemi.no>