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>

drivers/staging/comedi/comedi_compat32.h

@@ -37,7 +37,7 @@ extern long comedi_compat_ioctl(struct file *file, unsigned int cmd,
 
 #else /* CONFIG_COMPAT */
 
-#define comedi_compat_ioctl 0	/* NULL */
+#define comedi_compat_ioctl	NULL
 
 #endif /* CONFIG_COMPAT */