au1xxx-ide/rapide: use hwif->index for ide_unregister()

Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
diff --git a/drivers/ide/arm/rapide.c b/drivers/ide/arm/rapide.c
index e6b56d1..b5b6ed8 100644
--- a/drivers/ide/arm/rapide.c
+++ b/drivers/ide/arm/rapide.c
@@ -78,8 +78,8 @@
 
 	ecard_set_drvdata(ec, NULL);
 
-	/* there must be a better way */
-	ide_unregister(hwif - ide_hwifs);
+	ide_unregister(hwif->index);
+
 	ecard_release_resources(ec);
 }