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/dsp.c

@@ -855,6 +855,8 @@ void cnetz_set_dsp_mode(cnetz_t *cnetz, enum dsp_mode mode)
 {
 	PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "DSP mode %d -> %d\n", cnetz->dsp_mode, mode);
 	cnetz->dsp_mode = mode;
+	/* we must get rid of partly received frame */
+	fsk_demod_reset(&cnetz->fsk_demod);
 }
 
 void cnetz_set_sched_dsp_mode(cnetz_t *cnetz, enum dsp_mode mode, int frames_ahead)