Bluetooth: Move SMP (de)initialization to smp.c

As preparation for moving SMP to use l2cap_chan infrastructure we need
to move the (de)initialization functions to smp.c (where they'll
eventually need access to the local L2CAP channel callbacks).

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