Bluetooth: Fix returning proper mgmt status for Load LTKs

Failures of mgmt commands should be indicated with valid mgmt status
codes, and EINVAL is not one of them. Instead MGMT_STATUS_INVALID_PARAMS
should be returned.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>