Simplify elapsed time computation

Do as much of the arithmetic in integers as possible.
Convert both parts to final units(msec) and add.
The casts are actually unneeded, but show when the conversions are done.
And drop an unreachable break.

src/misc.c

@@ -1959,7 +1959,6 @@ double HAMLIB_API elapsed_ms(struct timespec *start, int option)
         //rig_debug(RIG_DEBUG_TRACE, "%s: after gettime, start = %ld,%ld\n", __func__,
         //          (long)start->tv_sec, (long)start->tv_nsec);
         return 999 * 1000; // so we can tell the difference in debug where we came from
-        break;
 
     case HAMLIB_ELAPSED_INVALIDATE:
         clock_gettime(CLOCK_REALTIME, start);
@@ -1968,8 +1967,8 @@ double HAMLIB_API elapsed_ms(struct timespec *start, int option)
         break;
     }
 
-    elapsed_msec = ((stop.tv_sec - start->tv_sec) + (stop.tv_nsec / 1e9 -
-                    start->tv_nsec / 1e9)) * 1e3;
+    elapsed_msec = (double)((stop.tv_sec - start->tv_sec) * 1000) + // sec -> ms
+                   (double)(stop.tv_nsec - start->tv_nsec) / 1e6;   // ns  -> ms
 
     //rig_debug(RIG_DEBUG_TRACE, "%s: elapsed_msecs=%.0f\n", __func__, elapsed_msec);