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/README

@@ -24,7 +24,7 @@ Here is a list of what each subdirectory here is, and what is contained in
 them.
 
 core/		- This is for the core USB host code, including the
-		  usbfs files and the hub class driver ("khubd").
+		  usbfs files and the hub class driver ("hub_wq").
 
 host/		- This is for USB host controller drivers.  This
 		  includes UHCI, OHCI, EHCI, and others that might