USB: of: clean up device-node helper

Clean up the USB device-node helper that is used to look up a device
node given a parent hub device and a port number. Also pass in a struct
usb_device as first argument to provide some type checking.

Give the helper the more descriptive name usb_of_get_device_node(),
which matches the new usb_of_get_interface_node() helper that is used to
look up a second type of of child node from a USB device.

Note that the terms "device node" and "interface node" are defined and
used by the OF Recommended Practice for USB.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>