Bluetooth: Remove unnecessary LTK type check from hci_add_ltk

All callers of hci_add_ltk pass a valid value to it. There are no places
where e.g. user space, the controller or the remote peer would be able
to cause invalid values to be passed. Therefore, just remove the
potentially confusing check from the beginning of the function.

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