Bluetooth: controller: Fix CSA#2 assert

Fix the assert in the controller during connection setup
when peer does not support CSA#2 feature and free Rx buffer
queue does not have enough buffers to generate CSA event.

The assert was reproduced by turning on advertisement
indication and scan request notification features in the
controllers advanced features, and a peer that does not
support CSA#2 initiated a connection.

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