usbcore: move usb_autosuspend_work

This patch (as864) moves the work routine for USB autosuspend from one
source file to another.  This permits the removal of one whole global
symbol (!) and should smooth the way for more changes in the future.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>