serial: core: Consider rs485 settings to drive RTS

Previously the rs485 settings weren't considered when setting the RTS
line, so e.g. closing and reopening a port made serial_core to drive
the line as if rs485 was disabled.

This patch fixes those issues.

Signed-off-by: Rafael Gago Castano <rgc@hms.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>