x86/microcode: Allow late microcode loading with SMT disabled

commit 07d981ad4cf1e78361c6db1c28ee5ba105f96cc1 upstream

The kernel unnecessarily prevents late microcode loading when SMT is
disabled.  It should be safe to allow it if all the primary threads are
online.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>