fbdev: hitfb: Use NULL for pointers

Replace integer constants with NULL. Resolves the following
warnings:

../drivers/video/fbdev/hitfb.c:447:23: warning: Using plain integer as NULL pointer
../drivers/video/fbdev/hitfb.c:465:23: warning: Using plain integer as NULL pointer

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Michael J. Ruhl <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/hitfb.c

···
 {
 	u16 v;
 
-	hitfb_blank(1,0);
+	hitfb_blank(1, NULL);
 	v = fb_readw(HD64461_STBCR);
 	v |= HD64461_STBCR_SLCKE_IST;
 	fb_writew(v, HD64461_STBCR);
···
 	v = fb_readw(HD64461_STBCR);
 	v &= ~HD64461_STBCR_SLCKE_IST;
 	fb_writew(v, HD64461_STBCR);
-	hitfb_blank(0,0);
+	hitfb_blank(0, NULL);
 
 	return 0;
 }