serial: 8250_dwlib: Implement SW half duplex support

This patch enables support for SW half-duplex mode using em485.

Cc: Eric Tremblay <etremblay@distech-controls.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220426122448.38997-3-ilpo.jarvinen@linux.intel.com


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