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>
diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index 7b166c2..b2b6f398 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -64,7 +64,7 @@
 /* #define DUMP_PACKETS */
 
 /*
- * how long to wait after some USB errors, so that khubd can disconnect() us
+ * how long to wait after some USB errors, so that hub_wq can disconnect() us
  * without too many spurious errors
  */
 #define ERROR_DELAY_JIFFIES (HZ / 10)