Bluetooth: BR: Improve `bt_conn_set_bondable`

In current, the bondable flag cannot be configured for each specific
BR connection.
But for LE conn, there is a function `bt_conn_set_bondable` for this
purpose.

Improve `bt_conn_set_bondable` to support BR conn.

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