USB: add kernel-doc for wusb_dev in struct usb_device

Reported by Randy Dunlap from a warning on the v2.6.29 merge window.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Cc: David Vrabel <david.vrabel@csr.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>