x86/microcode: Correct Kconfig dependencies

I have a randconfig here which has enabled only

  CONFIG_MICROCODE=y
  CONFIG_MICROCODE_OLD_INTERFACE=y

with both

  # CONFIG_MICROCODE_INTEL is not set
  # CONFIG_MICROCODE_AMD is not set

off. Which makes building the microcode functionality a little
pointless. Don't do that in such cases then.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1381682189-14470-1-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>