C-Netz: Allow SDR using C-Netz without calibration options

Crystals on SDRs are accurate enough for C-Netz (< 1ppm).

docs/c-netz.html

@@ -227,13 +227,15 @@ We need to calibrate the speed of our transmit signal (and receive signal as wel
 
 <p>
 <font color="red">Importaint: We need calibrate the signal processing clock!</font>
+<br><br>
+You can skip this parts, if you use SDR, because they have more accurate crystals than sound cards.
 </p>
 
 <p>
 First of all we need a reference clock.
 Since most computers have standard crystal clocks, we cannot use our system clock without any help.
 I decided to use the help of the NTP daemon.
-After running my system for several days (5 days), the clock speed was accurate enough for a base station.
+After running my system for one up to 5 days, the clock speed was accurate enough for a base station.
 Be sure to choose a place for your PC with constant temperature.
 A room inside a building is sufficient.
 </p>

src/cnetz/main.c

@@ -446,7 +446,7 @@ int main(int argc, char *argv[])
 		exit(0);
 	}
 
-	if (!set_clock_speed && !measure_speed) {
+	if (!set_clock_speed && !measure_speed && !use_sdr) {
 		printf("No clock speed given. You need to measure clock using '-M' and later correct clock using '-S <rx ppm>,<tx ppm>'. See documentation for help!\n\n");
 		mandatory = 1;
 	}