dm: core: Fix uninitialized return value from dm_scan_fdt_node

If there are no nodes or if all nodes are disabled, this function would
return err without setting it first. Fix this by initializing err to
zero.

Fixes: 94f7afdf ("dm: core: Ignore disabled devices when binding")

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>