libfdt: fdt_get_string(): Fix comparison warnings

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

In the first two cases, we have just established that the signed values
are not negative, so it's safe to cast the values to an unsigned type.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-7-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>