Fix FA21RF smoke detector detection

[andyboeh]

I've got a FA21RF smoke detector that was not properly detected by my ESP but works with the original RFLink. When comparing the raw signals, the original RFLink receives two more bytes at the beginning, but throws them away. The ESP RFLink does not receive these two bytes, unless SIGNAL_END_TIMEOUT_US is increased to about 10000. Since I expect side effects from that, I changed the FA21RF signal detection.

Transmission is untested as the transmitter behaves a bit weird on my Wemos D1 Mini. I'll investigate this later.