Bluetooth: L2CAP_BR: incorrect result returned if config opt unsupported

The `BT_L2CAP_CONF_SUCCESS` is returned as result for the config
req with supported opt.

Result `BT_L2CAP_CONF_UNKNOWN_OPT` should be returned for this case.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>