staging: comedi: ssv_dnp: remove empty private data

The private data struct 'dnp_private_data' contains no information.
Remove it and the alloc_private() call.

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