mISDN: use NULL pointer instead of plain integer

Fix more than 100 (all remaining in mISDN) sparse warnings:

  drivers/isdn/hardware/mISDN/hfcmulti.c:***:**: warning: Using plain integer as NULL pointer
  drivers/isdn/mISDN/dsp_tones.c:***:**: warning: Using plain integer as NULL pointer
  drivers/isdn/mISDN/dsp_pipeline.c:***:**: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/isdn/mISDN/dsp_pipeline.c b/drivers/isdn/mISDN/dsp_pipeline.c
index 850260a..5ee6651 100644
--- a/drivers/isdn/mISDN/dsp_pipeline.c
+++ b/drivers/isdn/mISDN/dsp_pipeline.c
@@ -249,7 +249,7 @@
 		name = strsep(&tok, "(");
 		args = strsep(&tok, ")");
 		if (args && !*args)
-			args = 0;
+			args = NULL;
 
 		list_for_each_entry_safe(entry, n, &dsp_elements, list)
 			if (!strcmp(entry->elem->name, name)) {