Bluetooth: Controller: Fix Data Length Update node Rx reservations

Reserve a minimum node rx of 2 that can happen when local
central initiated PHY Update reserves 2 node rx, one for PHY
Update complete and another for Data Length Update complete
notification. Otherwise, a peripheral only needs 1
additional node rx to generate Data Length Update complete
when PHY Update completes.

Relates to #36381.

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