dfu:fix: Replace wrong return value with proper one

This patch remove always false (since we tested ret = 0) ternary operator
with ret value returned.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
diff --git a/drivers/dfu/dfu.c b/drivers/dfu/dfu.c
index 8a09aaf..51b1026 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -219,7 +219,7 @@
 			ret = tret;
 	}
 
-	return ret = 0 ? size : ret;
+	return ret;
 }
 
 static int dfu_read_buffer_fill(struct dfu_entity *dfu, void *buf, int size)