Bluetooth: controller: Fix encryption setup to be queueable

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

The context related to encryption procedure is now shared so
that it will be used after the ongoing procedure completes.

The fix here maintains the old functionality of serializing
the queued data and LL Encryption Request PDU, so that data
queued before start encryption is acknowledged.

Fixes #15012.

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