C-Netz: Reset FSK decoder when switching channel mode

This prevents corrupt frames, as well as clear pending frames that
were received during the old mode, but may not be received during
the new mode.

src/cnetz/fsk_fm_demod.h

@@ -53,4 +53,5 @@ typedef struct fsk_fm_demod {
 int fsk_fm_init(fsk_fm_demod_t *fsk, cnetz_t *cnetz, int samplerate, double bitrate);
 void fsk_fm_demod(fsk_fm_demod_t *fsk, int16_t *samples, int length);
 void fsk_correct_sync(cnetz_t *cnetz, double offset);
+void fsk_demod_reset(fsk_fm_demod_t *fsk);