undoing a previous change

targets/ARCH/USRP/USERSPACE/LIB/usrp_lib.cpp

@@ -632,7 +632,7 @@ extern "C" {
     
     // do not explicitly set the clock to "internal", because this will disable the gpsdo
     //    // lock mboard clocks
-    s->usrp->set_clock_source("external");
+    //s->usrp->set_clock_source("external");
     // set master clock rate and sample rate for tx & rx for streaming
 
     device->type = USRP_B200_DEV;