libfdt: fdt_strerror(): Fix comparison warning

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

Force FDT_ERRTABSIZE to be signed (it's surely small enough to fit), so
that the types match. Also move the minus sign to errval, as this is
actually what we use in the next line.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-7-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>