fdtdec: correct test on return of fdt_node_offset_by_phandle

The result of fdt_node_offset_by_phandle is negative for error,
so this patch corrects the check of this result in
fdtdec_parse_phandle_with_args.

This patch allows to have the same behavior with or without OF_LIVE
for the function dev_read_phandle_with_args with cell_name = NULL and
with invalid phandle.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>