serial: rate limit custom-speed deprecation notice

Contrary to what a comment claimed, the ASYNC_SPD flags and custom
divisor can be set by a non-privileged user so rate limit the
deprecation notice as was intended.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>