staging: comedi: usbduxsigma: fix sparse endianness warnings

Sparse has many warnings like:

drivers/staging/comedi/drivers/usbduxsigma.c:293:21: warning: cast to
restricted __be32

on lines on which devpriv->in_buf is passed to be32_to_cpu().  This
suggests that this variable should actually be of type __be32.

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>