staging: comedi: comedi_compat_ioctl should be NULL not 0

When CONFIG_COMPAT is not defined, comedi_compat_ioctl should be
NULL not 0 in order to set the file_operations compat_ioctl variable
correctly.

This quiets a sparse warning about:

warning: Using plain integer as NULL pointer

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>