supplicant: display the MACsec CKN as hex string

The CKN can contain non-ASCII or NULL bytes. Display the original hex
string from configuration instead.

src/supplicant/nm-supplicant-config.c

@@ -458,7 +458,7 @@ nm_supplicant_config_add_setting_macsec(NMSupplicantConfig *self,
                                              "mka_ckn",
                                              (char *) buffer_ckn,
                                              sizeof(buffer_ckn),
-                                             NULL,
+                                             value,
                                              error))
             return FALSE;
     }