[MTD] assume mtd->writesize is 1 for NOR flashes

Signed-off-by: Artem B. Bityitskiy
diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 49bc9fdc..16a952dd 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -47,6 +47,7 @@
 {
 	int i;
 
+	BUG_ON(mtd->writesize == 0);
 	mutex_lock(&mtd_table_mutex);
 
 	for (i=0; i < MAX_MTD_DEVICES; i++)