usb: hub: rename khubd to hub_wq in documentation and comments

USB hub has started to use a workqueue instead of kthread. Let's update
the documentation and comments here and there.

This patch mostly just replaces "khubd" with "hub_wq". There are only few
exceptions where the whole sentence was updated. These more complicated
changes can be found in the following files:

	   Documentation/usb/hotplug.txt
	   drivers/net/usb/usbnet.c
	   drivers/usb/core/hcd.c
	   drivers/usb/host/ohci-hcd.c
	   drivers/usb/host/xhci.c

Signed-off-by: Petr Mladek <pmladek@suse.cz>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/tusb6010.c

@@ -433,7 +433,7 @@ static void musb_do_idle(unsigned long _musb)
 	if (!musb->is_active) {
 		u32	wakeups;
 
-		/* wait until khubd handles port change status */
+		/* wait until hub_wq handles port change status */
 		if (is_host_active(musb) && (musb->port1_status >> 16))
 			goto done;