x86-microcode: fix unbalanced use of get_cpu()

Don't use get_cpu() at all. Resort to checking a boot-up CPU (#0) in
microcode_{intel,amd}_module_init().

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>