Fix race condition causing repeated sign requests
Fix race condition causing repeated sign requests

The `break` only caused the inner loop to break, so the Transaction would
continue until the StateMachine itself is cancelled by the U2FManager,
which only happens when it is dropped.

This means that there would be a period in which the sign request is fully
done, but the manager is not yet dropped in which one or more other sign
requests can execute.

jcjones/u2f-hid-rs,jcjones/u2f-hid-rs