dfu, nand: before write a buffer to nand, erase the nand sectors

before writing the received buffer to nand, erase the nand
sectors. If not doing this, nand write fails. See for
more info here:

http://lists.denx.de/pipermail/u-boot/2013-June/156361.html

Using the nand erase option "spread", maybe overwrite
blocks on, for example another mtd partition, if the
erasing range contains bad blocks.
So a limit option is added to nand_erase_opts()

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
diff --git a/drivers/dfu/dfu_nand.c b/drivers/dfu/dfu_nand.c
index 7dc89b2..07dee89 100644
--- a/drivers/dfu/dfu_nand.c
+++ b/drivers/dfu/dfu_nand.c
@@ -63,12 +63,26 @@
 
 	nand = &nand_info[nand_curr_device];
 
-	if (op == DFU_OP_READ)
+	if (op == DFU_OP_READ) {
 		ret = nand_read_skip_bad(nand, start, &count, &actual,
 				lim, buf);
-	else
+	} else {
+		nand_erase_options_t opts;
+
+		memset(&opts, 0, sizeof(opts));
+		opts.offset = start;
+		opts.length = count;
+		opts.spread = 1;
+		opts.quiet = 1;
+		opts.lim = lim;
+		/* first erase */
+		ret = nand_erase_opts(nand, &opts);
+		if (ret)
+			return ret;
+		/* then write */
 		ret = nand_write_skip_bad(nand, start, &count, &actual,
 				lim, buf, 0);
+	}
 
 	if (ret != 0) {
 		printf("%s: nand_%s_skip_bad call failed at %llx!\n",