staging: comedi: usbduxfast: rename usb_device variables

For aesthetic reasons, globally rename the variables used for the
struct usb_device * in this driver to simply 'usb'.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>