Bluetooth: Classic: L2CAP: Discard the response with a invalid `ident`

In current implementation, the `ident` of the command response is not
checked whether it is same as the `ident` sent by the L2CAP command.

Keep the `ident` of the command. If the received `ident` of the
command response is not aligned with the `ident` of the command,
discard the response silently. Or, clear the saved `ident`.

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