libfdt: fdt_get_string(): Fix sequential write comparison warnings

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

Introduce a new usigned variable, which holds the actual (negated)
stroffset value, so we avoid negating all the other variables and have
proper types everywhere.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-6-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>