Bluetooth: Disconnect L2CAP connections without encryption

For L2CAP connections with high security setting, the link will be
immediately dropped when the encryption gets disabled. For L2CAP
connections with medium security there will be grace period where
the remote device has the chance to re-enable encryption. If it
doesn't happen then the link will also be disconnected.

The requirement for the grace period with medium security comes from
Bluetooth 2.0 and earlier devices that require role switching.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>