[Mac] Set a valid interface number on hid_device_info for USB HID devices

Previously the interface would never be set on Mac.

This presents a big pain because retrieving interface numbers can be the
only way to distinguish between the interfaces returned by HIDAPI.

Mac OS' IOKit library always returns interface number 0 UNLESS the
device is an HID device. If we know we have an HID device, we can safely
forward the interface number onto HIDAPI users.

This change makes it possible to retrieve interface number from an
hid_device_info on Mac for USB HID devices only.

Author: dylanmckay

hidapi/hidapi.h

@@ -69,9 +69,13 @@ extern "C" {
 			    (Windows/Mac only).*/
 			unsigned short usage;
 			/** The USB interface which this logical device
-			    represents. Valid on both Linux implementations
-			    in all cases, and valid on the Windows implementation
-			    only if the device contains more than one interface. */
+			    represents.
+
+				* Valid on both Linux implementations in all cases
+				* Valid on the Windows implementation only if the device
+				contains more than one interface
+				* Valid on the Mac implementation if and only if the device
+				* is a USB HID device. */
 			int interface_number;
 
 			/** Pointer to the next device */

mac/hid.c

@@ -25,6 +25,7 @@
 #include <IOKit/hid/IOHIDManager.h>
 #include <IOKit/hid/IOHIDKeys.h>
 #include <IOKit/IOKitLib.h>
+#include <IOKit/usb/USBSpec.h>
 #include <CoreFoundation/CoreFoundation.h>
 #include <wchar.h>
 #include <locale.h>
@@ -450,6 +451,17 @@ struct hid_device_info  HID_API_EXPORT *hid_enumerate(unsigned short vendor_id,
 			cur_dev->usage_page = get_int_property(dev, CFSTR(kIOHIDPrimaryUsagePageKey));
 			cur_dev->usage = get_int_property(dev, CFSTR(kIOHIDPrimaryUsageKey));
 
+			int device_class = get_int_property(dev, CFSTR(kUSBInterfaceClass));
+			bool is_hid = device_class == kUSBHIDClass;
+
+			// We can only retrieve the interface number for USB HID devices.
+			// IOKit always seems to return 0 when querying a standard USB device
+			// for its interface.
+			if (is_hid) {
+				/* Get the interface number */
+				cur_dev->interface_number = get_int_property(dev, CFSTR(kUSBInterfaceNumber));
+			}
+
 			/* Fill out the record */
 			cur_dev->next = NULL;