staging: comedi: usbduxsigma: fix more sparse endianness warnings

Sparse shows a couple of warnings like:

drivers/staging/comedi/drivers/usbduxsigma.c:787:23: warning: cast to restricted __be32

Looking at the indicated lines shows that the issue is caused by an
incorrect cast to uint32_t instead of __be32.  Fix this cast.

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