aw_cir: lower activation threshold to support NECx protocol

In NECx the leading mark has length of 8T as opposed to 16T in NEC,
where T is 562.5 us. So, 4.5 ms.
Our threshold was set to 128 * 42.7 us (derived from the sampling
frequency of 3/128 MHz). So, ~5.5 ms.

The new threshold is set to AW_IR_L1_MIN. I think that's a good enough
lower bound for detecting the leading pulse.

Also, calculations of active_delay (which is activation delay) are fixed.
Previously they would be wrong if AW_IR_ACTIVE_T was anything but zero,
because the value was already bit-shifted.

Finally, I am not sure why the activation delay was divided by two when
calculating the initial pulse length. I have not found anything that
would explain or justify it. So, I removed that division.

MFC after: 3 weeks