cleanup ftdi context if it was initialized

uses the internal structure of libftdi as seen in
https://www.intra2net.com/en/developer/libftdi/documentation/ftdi_8c_source.html#l00091
fixes #7

Author: l29ah

src/mpsse.c

@@ -237,7 +237,7 @@ void Close(struct mpsse_context *mpsse)
 {
 	if(mpsse)
 	{
-		if(mpsse->open)
+		if(mpsse->ftdi.eeprom)
 		{
 			ftdi_set_bitmode(&mpsse->ftdi, 0, BITMODE_RESET);
 			ftdi_usb_close(&mpsse->ftdi);