USB: Clarify that BusManagers do not really have usb_ids.

Instead their IDs are indexes into the stack, and are used primarily
for TRACE*() printing, so move the getter function into a "protected"
block and rename the variable appropriately.

src/add-ons/kernel/bus_managers/usb/BusManager.cpp

@@ -14,7 +14,7 @@ BusManager::BusManager(Stack *stack)
 	:	fInitOK(false),
 		fStack(stack),
 		fRootHub(NULL),
-		fUSBID((uint32)-1)
+		fStackIndex((uint32)-1)
 {
 	mutex_init(&fLock, "usb busmanager lock");
 
@@ -254,7 +254,7 @@ status_t
 BusManager::Start()
 {
 	fStack->AddBusManager(this);
-	fUSBID = fStack->IndexOfBusManager(this);
+	fStackIndex = fStack->IndexOfBusManager(this);
 	return B_OK;
 }
 

src/add-ons/kernel/bus_managers/usb/usb_private.h

@@ -226,9 +226,11 @@ virtual	status_t						NotifyPipeChange(Pipe *pipe,
 		Hub *							GetRootHub() const { return fRootHub; }
 		void							SetRootHub(Hub *hub) { fRootHub = hub; }
 
-		usb_id							USBID() const { return fUSBID; }
 virtual	const char *					TypeName() const = 0;
 
+protected:
+		usb_id							USBID() const { return fStackIndex; }
+
 protected:
 		bool							fInitOK;
 
@@ -245,7 +247,7 @@ private:
 		Hub *							fRootHub;
 		Object *						fRootObject;
 
-		usb_id							fUSBID;
+		usb_id							fStackIndex;
 };