dm: update test on of_offset in ofnode_valid

dm: update test on of_offset in ofnode_valid

Update the test for node.of_offset because an invalid offset is not
always set to -1 because the return value of the libfdt functions are:
+ an error with a value < 0
+ a valid offset with value >=0

For example, in ofnode_get_by_phandle() function, we have:
node.of_offset = fdt_node_offset_by_phandle(gd->fdt_blob, phandle);
and this function can return -FDT_ERR_BADPHANDLE (-6).

Without this patch, the added test dm_test_ofnode_get_by_phandle failed.

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