dfu: dfu_sf: Fix read offset

The offset was applied to write, but not read, now its applied to
both.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>

drivers/dfu/dfu_sf.c

@@ -20,7 +20,8 @@ static long dfu_get_medium_size_sf(struct dfu_entity *dfu)
 static int dfu_read_medium_sf(struct dfu_entity *dfu, u64 offset, void *buf,
 		long *len)
 {
-	return spi_flash_read(dfu->data.sf.dev, offset, *len, buf);
+	return spi_flash_read(dfu->data.sf.dev, dfu->data.sf.start + offset,
+		*len, buf);
 }
 
 static u64 find_sector(struct dfu_entity *dfu, u64 start, u64 offset)