Staging: comedi: fix sparse issues in comedi_test.c

This fixes all issues pointed out by sparse with the comedi_test.c
file.

Cc: Joachim Wuttke <Joachim.Wuttke@icn.siemens.de>
Cc: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/comedi/drivers/comedi_test.c

@@ -241,9 +241,9 @@ static int waveform_attach(comedi_device *dev, comedi_devconfig *it)
 	s->range_table = &waveform_ai_ranges;
 	s->len_chanlist = s->n_chan * 2;
 	s->insn_write = waveform_ao_insn_write;
-	s->do_cmd = 0;
-	s->do_cmdtest = 0;
-	s->cancel = 0;
+	s->do_cmd = NULL;
+	s->do_cmdtest = NULL;
+	s->cancel = NULL;
 
 	/* Our default loopback value is just a 0V flatline */
 	for (i = 0; i < s->n_chan; i++)