tty: use pr_warn_ratelimited() for deprecated serial flags

Use pr_warn_ratelimited() when warning about deprecated serial flags
instead of open coding.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407095208.31838-5-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>