serial.c: force DTR serial connection flag to ON for Windows clients

On Linux and MacOS the serial connection handler has the DTR flag
configured as ON by default. The Windows handler has it OFF by default,
which prevents a successful serial connection because the driver expects
it to be ON.
This issue can be reproduced using UART over USB.

Signed-off-by: AlexandraTrifan <Alexandra.Trifan@analog.com>

serial.c

@@ -335,6 +335,11 @@ static int apply_settings(struct sp_port *port, unsigned int baud_rate,
 {
 	int ret;
 
+#ifdef _WIN32
+	ret = libserialport_to_errno(sp_set_dtr(port, SP_DTR_ON));
+	if (ret)
+		return ret;
+#endif
 	ret = libserialport_to_errno(sp_set_baudrate(port, (int) baud_rate));
 	if (ret)
 		return ret;