Write a pure function for the table validation logic, it is
easier to unit-test.

Change-Id: I07b0f95ec0443fa6a8f54eb93f4a7ea1875cccad
Signed-off-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/37239


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>