Bluetooth: controller: Fix assert during DLE

Fix assert in controller checking ticker_id_prepare = 0 when
a scanner and connection are active, and the connection is
skipping events in order to resize Rx buffers.

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

subsys/bluetooth/controller/ll_sw/ctrl.c

@@ -8504,6 +8504,7 @@ static void event_connection_prepare(u32_t ticks_at_expire,
 				 * lets skip this event and try in the next
 				 * event.
 				 */
+				_radio.ticker_id_prepare = 0U;
 				return;
 			}
 #endif /* CONFIG_BT_CTLR_DATA_LENGTH */