dfu: dfu_sf: Read the SPI flash in 16 MiB chunks

Not all SPI flashes and controllers can do continuous transfer longer
than 16 MiB, so perform the DFU read in 16 MiB chunks.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/drivers/dfu/dfu_sf.c b/drivers/dfu/dfu_sf.c
index 7e64ab7..b72493c 100644
--- a/drivers/dfu/dfu_sf.c
+++ b/drivers/dfu/dfu_sf.c
@@ -24,8 +24,18 @@
 static int dfu_read_medium_sf(struct dfu_entity *dfu, u64 offset, void *buf,
 			      long *len)
 {
-	return spi_flash_read(dfu->data.sf.dev, dfu->data.sf.start + offset,
-		*len, buf);
+	long seglen = *len;
+	int ret;
+
+	if (seglen > (16 << 20))
+		seglen = (16 << 20);
+
+	ret = spi_flash_read(dfu->data.sf.dev, dfu->data.sf.start + offset,
+		seglen, buf);
+	if (!ret)
+		*len = seglen;
+
+	return ret;
 }
 
 static u64 find_sector(struct dfu_entity *dfu, u64 start, u64 offset)