Bluetooth: Simplify hci_conn_accept_secure check

If the link key is secure (authenticated or combination 16 digit)
the sec_level will be always BT_SECURITY_HIGH. Therefore, instead
of checking the link key type simply check the sec_level on the link.

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