Add master clock rate display to GUI.

Calculate full range of sample rates supported, by varying master clock.
Check if requested sample rate can be met.

plugins/samplesource/usrpinput/usrpinputsettings.cpp

@@ -26,6 +26,7 @@ USRPInputSettings::USRPInputSettings()
 
 void USRPInputSettings::resetToDefaults()
 {
+    m_masterClockRate = -1; // Calculated by UHD
     m_centerFrequency = 435000*1000;
     m_devSampleRate = 3000000;
     m_loOffset = 0;