Extract interface numbers into enums.

Bootloaders/MassStorage/Descriptors.h

@@ -68,6 +68,16 @@
 			USB_Descriptor_Endpoint_t             MS_DataOutEndpoint;
 		} USB_Descriptor_Configuration_t;
 
+
+		/** Enum for the device interface descriptor IDs within the device. Each interface descriptor
+		 *  should have a unique ID index associated with it, which can be used to refer to the
+		 *  interface from other descriptors.
+		 */
+		enum InterfaceDescriptors_t
+		{
+			INTERFACE_ID_MassStorage = 0, /**< Mass storage interface descriptor ID */
+		};
+
 	/* Function Prototypes: */
 		uint16_t CALLBACK_USB_GetDescriptor(const uint16_t wValue,
 		                                    const uint8_t wIndex,