NRF5x: Add bug fix for the first timer read.

It has been noticed that first read value can not be trusted.

targets/TARGET_NORDIC/TARGET_NRF5x/us_ticker.c

@@ -100,6 +100,9 @@ void us_ticker_init(void)
 
     nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_START);
 
+    /* Bug fix. First value can't be trusted. */
+    nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_CAPTURE1);
+
     us_ticker_initialized = true;
 }