mtd: powernv_flash: Use WARN_ON_ONCE() rather than BUG_ON()

BUG_ON() should be reserved in situations where we can not longer
guarantee the integrity of the system. In the case where
powernv_flash_async_op() receives an impossible op, we can still
guarantee the integrity of the system.
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/mtd/devices/powernv_flash.c

@@ -78,7 +78,9 @@ static int powernv_flash_async_op(struct mtd_info *mtd, enum flash_op op,
 		rc = opal_flash_erase(info->id, offset, len, token);
 		break;
 	default:
-		BUG_ON(1);
+		WARN_ON_ONCE(1);
+		opal_async_release_token(token);
+		return -EIO;
 	}
 
 	if (rc != OPAL_ASYNC_COMPLETION) {