SCARD_ATTR_CHANNEL_ID: use port number instead of device address

The device address field is a number that is increasing after each USB
device plug-in. It is NOT related to a physical USB port.

It is the Device: number returned by lsusb(1):
Bus 001 Device 021: ID 08e6:3437 Gemalto (was Gemplus) GemPC Twin SmartCard Reader

The port number is related to a USB physical port and is stable if you
unplug and replug a USB device on the same port.

According to libusb_get_port_number() API documentation:
  Get the number of the port that a device is connected to. Unless the
  OS does something funky, or you are hot-plugging USB extension cards,
  the port number returned by this call is usually guaranteed to be
  uniquely tied to a physical port, meaning that different devices
  plugged on the same physical port should return the same port number.

  But outside of this, there is no guarantee that the port number
  returned by this call will remain the same, or even match the order in
  which ports have been numbered by the HUB/HCD manufacturer.

src/ccid_usb.c

@@ -81,6 +81,7 @@ typedef struct
  uint8_t bus_number;
  uint8_t device_address;
  int interface;
+ uint8_t port_number;
 
  /*
  * Endpoints
@@ -697,6 +698,7 @@ status_t OpenUSBByName(unsigned int reader_index, /*@null@*/ char *device)
  usbDevice[reader_index].real_nb_opened_slots = 1;
  usbDevice[reader_index].nb_opened_slots = &usbDevice[reader_index].real_nb_opened_slots;
  usbDevice[reader_index].polling_transfer = NULL;
+ usbDevice[reader_index].port_number = libusb_get_port_number(dev);
 
  /* CCID common informations */
  usbDevice[reader_index].ccid.real_bSeq = 0;
@@ -1104,7 +1106,7 @@ EXTERNAL uint8_t get_ccid_usb_bus_number(int reader_index) {
 }
 
 EXTERNAL uint8_t get_ccid_usb_device_address(int reader_index) {
- return usbDevice[reader_index].device_address;
+ return usbDevice[reader_index].port_number;
 }
 
 /*****************************************************************************