Bluetooth: Add paranoid check for existing LE and BR/EDR SMP channels

When the SMP channels have been already registered, then print out a
clear WARN_ON message that something went wrong. Also unregister the
existing channels in this case before trying to register new ones.

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