Bluetooth: Allow 3D profile to use security mode 4 level 0

The PSM 0x0021 is dedicated to the 3D profile and has permission to
use security mode 4 level 0 for L2CAP connectionless unicast data
transfers.

When establishing a L2CAP connectionless channel on PSM 0x0021, it
will no longer force Secure Simple Pairing.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>