staging: comedi: usbduxsigma: fix kcalloc 'size' argument

This code wants to allocate two arrays of struct urb pointers not two
arrays of struct urb objects. The size argument for both kcalloc calls
should be sizeof(urb) not sizeof(*urb).

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