EFM32: fix fire interrupt (set flags)

There's overflow counter that needs to be 0, and
CC0 flag set.

Fixes #5051

Author: 0xc0170

targets/TARGET_Silicon_Labs/TARGET_EFM32/us_ticker.c

@@ -213,6 +213,8 @@ void us_ticker_set_interrupt(timestamp_t timestamp)
 
 void us_ticker_fire_interrupt(void)
 {
+    ticker_int_cnt = 0;
+    TIMER_IntSet(US_TICKER_TIMER, TIMER_IF_CC0);
     NVIC_SetPendingIRQ(US_TICKER_TIMER_IRQ);
 }