libfdt: fdt_node_offset_by_phandle(): Fix comparison warning

With -Wsign-compare, compilers warn about a mismatching signedness
in a comparison in fdt_node_offset_by_phandle().

Uses a better suited bitwise NOT operator to denote the special value of
-1, which automatically results in an unsigned type.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-14-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>