[MTD] Introduce writesize

At least two flashes exists that have the concept of a minimum write unit,
similar to NAND pages, but no other NAND characteristics.  Therefore, rename
the minimum write unit to "writesize" for all flashes, including NAND.

Signed-off-by: Joern Engel <joern@wh.fh-wedel.de>
diff --git a/drivers/mtd/nand/au1550nd.c b/drivers/mtd/nand/au1550nd.c
index d9a0143..4253b93 100644
--- a/drivers/mtd/nand/au1550nd.c
+++ b/drivers/mtd/nand/au1550nd.c
@@ -356,9 +356,9 @@
 	if (command == NAND_CMD_SEQIN) {
 		int readcmd;
 
-		if (column >= mtd->oobblock) {
+		if (column >= mtd->writesize) {
 			/* OOB area */
-			column -= mtd->oobblock;
+			column -= mtd->writesize;
 			readcmd = NAND_CMD_READOOB;
 		} else if (column < 256) {
 			/* First 256 bytes --> READ0 */