rx: reduce squelch threshold from 10 to 6 dB

During tests it was determined that this is much more reliable. The theory is
that the first packet of a burst was frequently not detected because the
squelch opened too late if the channel was very noisy.

Unfortunately, the squelch now opens very often even if no signal is present.
This will be improved in the future.

impl/src/layer1/rx.c

@@ -206,7 +206,7 @@ static enum squelch_state_t update_and_check_squelch(layer1_rx_t *rx, unsigned i
 			// Adjustment value is in dB.
 			rx->noise_floor_level += 1e-4f;
 		}
-		agc_crcf_squelch_set_threshold(rx->agc, rx->noise_floor_level + 10.0f); // in dB
+		agc_crcf_squelch_set_threshold(rx->agc, rx->noise_floor_level + 6.0f); // in dB
 	}
 
 	switch(agc_crcf_squelch_get_status(rx->agc)) {