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/misc/usbtest.c

@@ -2031,7 +2031,7 @@ static int test_unaligned_bulk(
  *
  * WARNING:  Because usbfs grabs udev->dev.sem before calling this ioctl(),
  * it locks out usbcore in certain code paths.  Notably, if you disconnect
- * the device-under-test, khubd will wait block forever waiting for the
+ * the device-under-test, hub_wq will wait block forever waiting for the
  * ioctl to complete ... so that usb_disconnect() can abort the pending
  * urbs and then call usbtest_disconnect().  To abort a test, you're best
  * off just killing the userspace task and waiting for it to exit.