zephyr: adapt to changes in flash_img.c

Update member access of context.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
diff --git a/cmd/img_mgmt/port/zephyr/src/zephyr_img_mgmt.c b/cmd/img_mgmt/port/zephyr/src/zephyr_img_mgmt.c
index af9abbb..e5db289 100644
--- a/cmd/img_mgmt/port/zephyr/src/zephyr_img_mgmt.c
+++ b/cmd/img_mgmt/port/zephyr/src/zephyr_img_mgmt.c
@@ -292,7 +292,7 @@
 		}
 	}
 
-	if (offset != ctx->bytes_written + ctx->buf_bytes) {
+	if (offset != ctx->stream.bytes_written + ctx->stream.buf_bytes) {
 		return MGMT_ERR_EUNKNOWN;
 	}