libfdt: fdt_splice_(): Fix comparison warning

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

Since we just established that oldlen is not negative, we can safely
cast it to an unsigned type.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-8-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>