fbdev: fix integer as NULL pointer warning

drivers/video/aty/atyfb_base.c:3359:26: warning: Using plain integer as NULL pointer
drivers/video/aty/radeon_base.c:2280:32: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/sis/sis_main.c:5790:44: 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>
diff --git a/drivers/video/sis/sis_main.c b/drivers/video/sis/sis_main.c
index 7380362..b934384 100644
--- a/drivers/video/sis/sis_main.c
+++ b/drivers/video/sis/sis_main.c
@@ -5787,7 +5787,7 @@
 	} else {
 		struct sis_video_info *countvideo = card_list;
 		ivideo->cardnumber = 1;
-		while((countvideo = countvideo->next) != 0)
+		while((countvideo = countvideo->next) != NULL)
 			ivideo->cardnumber++;
 	}