Documentation: rs485: Mention uart_get_rs485_mode()

Documentation: rs485: Mention uart_get_rs485_mode()

Add to rs485 documentation that serial core prepares the struct
serial_rs485 when uart_get_rs485_mode() is called. Remove the wrong
claim that the driver must fill it by itself.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221019093343.9546-4-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>