RSSI calculations are incorrect

[terrillmoore]

The RSSI calculation doesn't take into account difference between SX1272 and SX1276, nor the formulas given in section 5.5.5 of the respective datasheets.

[terrillmoore]

Still broken based on testing.

[terrillmoore]

I have a fix that gives results consistent with external attenuators. Will bring onto the development branch.