Bluetooth: Reset the security level after an authentication failure

When authentication fails for a connection the assumed security level
should be set back to BT_SECURITY_LOW so that subsequent connect
attempts over the same link don't falsely assume that security is
adequate enough.

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