[stm32f0xx] Fix type of overflow counter.

stm32_timer_calc_value was using uint32_t for the overflow counter,
ultimately causing timestamps to wrap as uint32_t.

platform/stm32f0xx/timer_capture.c

@@ -222,7 +222,7 @@ static uint32_t stm32_timer_median_val(stm32_timer_capture_t *tc) {
 
 // Assumes interrupts are disabled.
 static uint64_t stm32_timer_calc_value(stm32_timer_capture_t *tc, uint32_t sr, uint32_t val) {
-    uint32_t overflow = tc->overflow;
+    uint64_t overflow = tc->overflow;
     // Since we could be processing an overflow and capture interrupts at the
     // same time, we don't know the ordering of the two.  Here we assume
     // that if the capture event occurred in the lower half of the counter