libfdt: fdt_offset_ptr(): Fix comparison warnings

With -Wsign-compare, compilers warn about mismatching signedness in
comparisons in fdt_offset_ptr().

This mostly stems from "offset" being passed in as a signed integer,
even though the function would not really tolerate negative values.

Short of changing the prototype, check that offset is not negative, and
use an unsigned type internally.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-2-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>