fix(allwinner): dtb: check for correct error condition

In sunxi_soc_fdt_fixup(), we check for the value of "ret" again, after
calling fdt_node_offset_by_phandle(), even though the error value of
that lands in "node".

Check for "node" being non-negative instead, to properly detect any
errors here.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I57c1406388dbe11d343038da173019519e18af3e