Bluetooth: conn: move auto-init procedures to system workqueue

`conn_auto_initiate()` starts a bunch of controller procedures (read: HCI
commands) that are fired off right after connection establishment.

Right now, it's called from the RX context, which is the same context where
resources (cmd & acl buffers) are freed. This not ideal.

But the procedures are all async, so it should be fine to schedule this
function on the system workqueue, where we have less risk of deadlocks.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>