mac802154: mac802154_mlme_start_req() optimisation.

mac802154_mlme_start_req() calls
ieee802154_mlme_ops(dev)->llsec->set_params() on the net_device
passed into it, however, this net_device will always be a mac802154
net_device, so just call mac802154_set_params() directly instead.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>