Replace separate M10/M20 decoders with combined decoder

[darksidelemm]

From here: https://github.com/rs1729/RS/blob/test/demod/mod/m10m20mod.c

May help with M10/M20 mis-detection.

Currently we are running the decoders with the following parameters:

M10:

            _baud_rate = 9616
            _sample_rate = 48080
            _p = 5 # Override the oversampling rate

            # Limit FSK estimator window to roughly +/- 10 kHz
            _lower = -10000
            _upper = 10000

M20:

            _baud_rate = 9600
            _sample_rate = 48000
            _p = 5

            # Limit FSK estimator window to roughly +/- 10 kHz
            _lower = -10000
            _upper = 10000

If we just pick 9600 baud for the combined decoder, then any M10 sondes running at 9616 baud will have an effective 1666 ppm symbol rate error. This shouldn't detrimentally impact fsk_demod performance (It takes about +/- 3000 ppm symbol rate error to see degradation in performance).