Bluetooth: Include security level 4 in connections check

This check is only used for RFCOMM connections and most likely no
RFCOMM based profile will require security level 4 secure connection
security policy. In case it ever does make sure that seucrity level 4
is treated as sufficient security level.

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