serial: dz: Assume previous baudrate is valid

Assume previously used termios has a valid baudrate and use
it directly.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220816115739.10928-4-ilpo.jarvinen@linux.intel.com


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