SATA: use NULL for ptrs

Fix sparse warnings in SATA:
drivers/ata/sata_sil.c:342:9: warning: Using plain integer as NULL pointer
drivers/ata/sata_mv.c:2056:55: warning: Using plain integer as NULL pointer

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/drivers/ata/sata_sil.c b/drivers/ata/sata_sil.c
index dca3d37..1487255 100644
--- a/drivers/ata/sata_sil.c
+++ b/drivers/ata/sata_sil.c
@@ -339,7 +339,7 @@
 		break;
 	}
 
-	return 0;
+	return NULL;
 }
 
 static u32 sil_scr_read (struct ata_port *ap, unsigned int sc_reg)