libfdt: libfdt_wip: Fix comparison warning

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

fdt_getprop_namelen() will only return negative error values in "proplen"
if the return value is NULL. So we can rely on "proplen" being positive
in our case and can safely cast it to an unsigned type.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-5-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>