serial: cpm_uart: Remove custom frame size calculation

The number of bits can be calculated using helpers in core, no need for
the driver to do it on its own.

The mode register is programmed with frame bits minus 1, rearrange the
comments related to that "feature" closer to the actual write.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220830084925.5608-3-ilpo.jarvinen@linux.intel.com


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