of/usb: fix build error due to of_node pointer move

Fix driver to use new location of of_node pointer (introduced by commit
use new location of of_node pointer (introduced by commit
61c7a080; of: Always use 'struct
device.of_node' to get device node pointer)

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reported-by: John Linn <john.linn@xilinx.com>
CC: Greg Kroah-Hartman <gregkh@suse.de>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: linux-usb@vger.kernel.org
CC: devicetree-discuss@lists.ozlabs.org