Channel check interval is clock_time_t

Fix channel_check_interval routine in struct rdc_driver to be of type
clock_time_t -- otherwise this may overflow on some architectures.

examples/osd/native-border-router/border-router-rdc.c

@@ -170,7 +170,7 @@ off(int keep_radio_on)
   return 1;
 }
 /*---------------------------------------------------------------------------*/
-static unsigned short
+static clock_time_t
 channel_check_interval(void)
 {
   return 0;