Bluetooth: controller: Reserve an additional node rx for Ext. Initiator

Reserve an additional node rx buffer when Extended Initiator
is supported as the received ADV_EXT_IND PDU is being
buffered in the auxiliary channel scanning instance and is
only released/flushed in the done event of the initiating
auxiliary channel radio event.

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