isdn: fix integer as NULL pointer warning

drivers/isdn/hysdn/hycapi.c:465:42: warning: Using plain integer as NULL pointer
drivers/isdn/hysdn/hycapi.c:467:44: warning: Using plain integer as NULL pointer
drivers/isdn/hysdn/hycapi.c:469:42: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/hysdn/hycapi.c

@@ -462,11 +462,11 @@ static int hycapi_read_proc(char *page, char **start, off_t off,
 		default: s = "???"; break;
 	}
 	len += sprintf(page+len, "%-16s %s\n", "type", s);
-	if ((s = cinfo->version[VER_DRIVER]) != 0)
+	if ((s = cinfo->version[VER_DRIVER]) != NULL)
 		len += sprintf(page+len, "%-16s %s\n", "ver_driver", s);
-	if ((s = cinfo->version[VER_CARDTYPE]) != 0)
+	if ((s = cinfo->version[VER_CARDTYPE]) != NULL)
 		len += sprintf(page+len, "%-16s %s\n", "ver_cardtype", s);
-	if ((s = cinfo->version[VER_SERIAL]) != 0)
+	if ((s = cinfo->version[VER_SERIAL]) != NULL)
 		len += sprintf(page+len, "%-16s %s\n", "ver_serial", s);
 
 	len += sprintf(page+len, "%-16s %s\n", "cardname", cinfo->cardname);