x86: microcode_amd: fix wrong handling of equivalent CPU id

Impact: fix bug resulting in non-loaded AMD microcode

mc_header->processor_rev_id is a 2 byte value. Similar is true for
equiv_cpu in an equiv_cpu_entry -- only 2 bytes are of interest.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>