usb: hub: rename usb_kick_khubd() to usb_kick_hub_wq()

USB hub started to use a workqueue instead of kthread. Let's make it clear from
the function names.

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>