fix rdtsc usage

One user had a problem compiling oaisim.
This commit fixes it.

The compilation log was saying:

    targets/RT/USER/lte-ue.c:
    In function UE_thread_rxn_txnp4:

    openair2/UTIL/LOG/log.h:370:3:
    error: inconsistent operand constraints in an asm

         __asm__ volatile ("rdtsc" : "=a" (a), "=d" (d));

         ^

openair2/UTIL/LOG/log.h

@@ -373,9 +373,9 @@ void *log_thread_function(void * list);
 /* @}*/
 
 static __inline__ uint64_t rdtsc(void) {
-  uint64_t a, d;
+  uint32_t a, d;
   __asm__ volatile ("rdtsc" : "=a" (a), "=d" (d));
-  return (d<<32) | a;
+  return (((uint64_t)d)<<32) | ((uint64_t)a);
 }
 
 #define DEBUG_REALTIME 1