Bluetooth: Classic: Check LK before clearing it

After SSP has been completed and before LK event notification, the
link key may be invalid when handling the ACL disconnection event.

Check `conn->br.link_key` before calling the function
`bt_keys_link_key_clear()` to clear it.

Fixes #87880.

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