x86: Correct problems in the microcode loading

There are several problems in the code. The device tree decode is incorrect
in ways that are masked due to a matching bug. Both are fixed. Also
microcode_read_rev() should be inline and called before the microcode is
written.

Note: microcode writing does not work correctly on ivybridge for me. Further
work is needed to resolve this. But this patch tidies up the existing code
so that will be easier.

Signed-off-by: Simon Glass <sjg@chromium.org>