Low power ticker wrapper fix

When the Timeout object has been setup to perform the next low power
ticker write don't write to the low power ticker from
lp_ticker_set_interrupt_wrapper. Instead write to it only from
set_interrupt_later. This prevents a race condition where the low
power ticker is written to in two places.

Author: c1728p9

hal/mbed_lp_ticker_wrapper.cpp

@@ -122,7 +122,7 @@ void lp_ticker_set_interrupt_wrapper(timestamp_t timestamp)
         }
     }
 
-    if (pending) {
+    if (pending || timeout_pending) {
         next = timestamp;
         last_request = current;
         if (!timeout_pending) {