x86/microcode/AMD: Move the no-revision fixup to get_patch_level()

x86/microcode/AMD: Move the no-revision fixup to get_patch_level()

On machines which don't have microcode applied yet, the revision is 0.
However, this doesn't work with the Zen family/model/stepping patch
arithmetic. So move the fixup to the patch level getter function and
this way make sure the patch level is always proper and thus the
arithmetic always works.

And now that it can be called on any family, make this Zen-only.

Assisted-by: claude/claude-opus-4-6
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20260530024213.86137-1-bp@kernel.org