nimble/ll: Fix recalculating rfclk start

ble_ll_xcvr_rfclk_timer_start() assumes that it should not reschedule
rfclk if it's already scheduled before requested tick. This is correct
since it may be called it different contexts, however when called from
ble_ll_sched_rfclk_chk_restart() we expect that rfclk will be scheduled
for a first item in scheduler and this may not work as expected.

Consider following scenario (one of many):
- connection is active
- sync with periodic advertising is active
- scaner is active
- sync scan item is in scheduler
- next connection event item is scheduled *after* above sync scan item

At this point we have rfclk timer set to fire at sync scan item. And now
interesting things will happen:
- terminate sync which removes sync scan item from scheduler
- disable scan

If both of the above happen before scheduled connection event item, we
have a problem. This is because when sync scan item was removed, we did
not move rfclk timer to connection event item because rfclk code checked
that it already has rfclk scheduled before requested time. This means
rfclk will be enabled at the time of removed sync scan item, but it will
not be rescheduled to connection event item since there is no scheduler
item do do this. So far so good - we still have rfclk enabled. But then
we have scan disable which disables rfclk since connection event item is
apparently far enough in the future so this is allowed. As a result we
do not have rfclk enabled for a connection event.

To fix this we make sure that every time we reevaluate scheduler for
rfclk change, we need to stop existing timer so it can be always started
for a proper item.

nimble/controller/src/ble_ll_sched.c

@@ -1608,6 +1608,15 @@ ble_ll_sched_rfclk_chk_restart(void)
          */
         time_till_next = (int32_t)(next_time - os_cputime_get32());
         if (time_till_next > g_ble_ll_data.ll_xtal_ticks) {
+            /*
+             * XXX stop timer if already scheduled since it could be set for a
+             *     scheduler item which was removed prior to calling this func.
+             *     in such case we need to make sure we are set to proper item.
+             */
+            if (g_ble_ll_data.ll_rfclk_is_sched) {
+                g_ble_ll_data.ll_rfclk_is_sched = 0;
+                os_cputime_timer_stop(&g_ble_ll_data.ll_rfclk_timer);
+            }
             /* Restart the rfclk timer based on the next scheduled time */
             ble_ll_xcvr_rfclk_timer_start(next_time);