part: Fix bogus return from part_get_info_by_dev_and_name

blk_get_device_by_str returns the device number on success. So we must
check if the return was negative to determine an error.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/disk/part.c b/disk/part.c
index 80ced2b..5e7e59c 100644
--- a/disk/part.c
+++ b/disk/part.c
@@ -725,7 +725,7 @@
 	}
 
 	ret = blk_get_device_by_str(dev_iface, dev_str, dev_desc);
-	if (ret)
+	if (ret < 0)
 		goto cleanup;
 
 	ret = part_get_info_by_name(*dev_desc, part_str, part_info);