Bluetooth: controller: split: Port Enc setup to be queueable

Port the fix for the controller implementation to make start
encryption queueable if there is any control procedure in
progress.

Refer to #15335.
Relates to #15335, #15186, #15958 and #14636.

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