kernel: sched: Fix comparsion in _update_time_slice_before_swap()

The _update_time_slice_before_swap() function directly compared
_time_slice_duration (expressed in ms) with value returned by
_get_remaining_program_time() which used ticks as a time unit.

Moreover, the _time_slice_duration was also used as an argument
for _set_time(), which expects time expressed in ticks.

This commit ensures that the same unit (ticks) is used in
comparsion and timer adjustments.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>

Author: pizi-nordic

kernel/sched.c

@@ -641,9 +641,10 @@ void _update_time_slice_before_swap(void)
 	}
 
 	u32_t remaining = _get_remaining_program_time();
+	u32_t time_slice_ticks = _ms_to_ticks(_time_slice_duration);
 
-	if (!remaining || (_time_slice_duration < remaining)) {
-		_set_time(_time_slice_duration);
+	if (!remaining || (_time_slice_ticks < remaining)) {
+		_set_time(_time_slice_ticks);
 	} else {
 		/* Account previous elapsed time and reprogram
 		 * timer with remaining time