mbr: Correct verification check

At present this command considers the partitions to be identical if the
start and size are smaller than expected. It should check that they are
the same. Fix this and tidy up the code style a little.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>