dm: core: Fix offset_to_ofnode() with invalid offset

If the offset is -1 this function correctly sets up a null ofnode. But if
the offset is any other negative number (e.g. -FDT_ERR_BADPATH) then it
does the wrong thing.

An offset of -1 in ofnode indicates that the ofnode is not valid. Any
other negative value is not handled by ofnode_valid(). We could of course
change that function, but it seems much better to always use the same
value for an invalid node.

Fix it by setting the offset to -1 if it is invalid for any reason.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>