serial: 8250_lpss: Extract dw8250_do_set_termios() for common use

serial: 8250_lpss: Extract dw8250_do_set_termios() for common use

Some of the code currently used in dw8250_set_termios(), byt_set_termios()
may be reused by other methods in the future. Extract it to a common helper
function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211005133026.21488-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>