Bluetooth: Controller: Add verbose assertion on delayed prepare

When an event's prepare is delayed due to ULL scheduling or
ISR latencies, check and have a verbose assertion that logs
the actual EVENT_OVERHEAD_START_US required.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

subsys/bluetooth/controller/ll_sw/nordic/lll/lll.c

@@ -472,10 +472,13 @@ uint32_t lll_preempt_calc(struct ull_hdr *ull, uint8_t ticker_id,
 		 *    duration.
 		 * 3. Increase the preempt to start ticks for future events.
 		 */
-		return 1;
+		LL_ASSERT_MSG(false, "%s: Actual EVENT_OVERHEAD_START_US = %u",
+			      __func__, HAL_TICKER_TICKS_TO_US(diff));
+
+		return 1U;
 	}
 
-	return 0;
+	return 0U;
 }
 
 void lll_chan_set(uint32_t chan)