Bluetooth: controller: split: Fix aborting of connection req PDU

Fix the initiator so that connection request PDU is not abort
mid-air by preemption by the overalapping first connection
event.

If the connection establishment is in progress, then the
first connection event trying to abort the initiator will
wait the connection request to be transmited completely.

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