Bluetooth: Add SMP-internal timeout callback

This patch adds an SMP-internal timeout callback to remove the depenency
on (the soon to be removed) l2cap_conn->security_timer. The behavior is
the same as with l2cap_conn->security_timer except that the new
l2cap_conn_shutdown() public function is used instead of the L2CAP core
internal l2cap_conn_del().

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