Bluetooth: Fix auth_complete_evt for legacy units

Legacy devices don't re-authenticate the link properly if a link key
already exists.  Thus, don't update sec_level for this case even if
hci_auth_complete_evt indicates success. Otherwise the sec_level will
not reflect a real security on the link.

Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>