descrambler: improve the ECM handling

- for constcw (CAID 2600) do not expect any key change
- when the stream is broken, try to issue new ECM request

Note: The TS data might be wrong (which may cause the ECM late
situations). This patch tries to ask for new ECM to be sure
that the keys are correct. The constcw specific code is just
an optimization. The new ECM request is not required for it.