Bluetooth: Host: Notify L2CAP that security change failed

Notify L2CAP when pairing procedure fails during SMP pairing
process. L2CAP needs to be notified so that it can cancel
ongoing connection-oriented channel requests.

Rename bt_l2cap_encrypt_change to bt_l2cap_security_changed and
call it from bt_conn_security_changed always, so that L2CAP
is informed when changing security fails.

This also fixes security_changed conn callback not called when
auth_complete returns an error.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>