dfu: fix readback buffer overflow test

The buffer is too small if it's < size to read, not if it's <= the size.
This fixes the 1MB test case on Tegra, which has a 1MB buffer.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
diff --git a/drivers/dfu/dfu.c b/drivers/dfu/dfu.c
index 6cd3fbb..3512b14 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -332,7 +332,7 @@
 		case DFU_RAM_ADDR:
 			break;
 		default:
-			if (dfu->r_left >= dfu_buf_size) {
+			if (dfu->r_left > dfu_buf_size) {
 				printf("%s: File too big for buffer\n",
 				       __func__);
 				return -EOVERFLOW;