usb: wusbcore: fix build warning on 64-bit builds

Fix a build warning found by the kbuild test robot in the most recent
wusbcore patches.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>