staging: comedi: usbduxsigma: tidy up usbduxsigma_usb_probe()

For aesthetic reasons, rename some of the variables to the "norm" used
in comedi drivers.

Use a local variable for the pointer to the private data instead of
accessing the static array directly. Also use a local variable to
setup the urbs.
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>