fixes an issue with clock source

- command line paramter --clock-source can only be set at UE (removed this option from gNB)
 - at gNB use clock_source from RU section in config file
 - in both cases if clock_source is present in usrp_args, it will override clock_source

targets/ARCH/COMMON/common_lib.h

@@ -135,6 +135,8 @@ typedef struct {
 
 /*! \brief Clock source types */
 typedef enum {
+  //! this means the paramter has not been set
+  unset=-1,
   //! This tells the underlying hardware to use the internal reference
   internal=0,
   //! This tells the underlying hardware to use the external reference