serial: Move serial_rs485 sanitization into separate function

Add uart_sanitize_serial_rs485() function for sanitizing serial_rs485
structure fields.

No functional changes.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606100433.13793-3-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>