x86, microcode, AMD: Fix broken ucode patch size check

This issue was recently observed on an AMD C-50 CPU where a patch of
maximum size was applied.

Commit be62adb4

 ("x86, microcode, AMD: Simplify ucode verification")
added current_size in get_matching_microcode(). This is calculated as
size of the ucode patch + 8 (ie. size of the header). Later this is
compared against the maximum possible ucode patch size for a CPU family.
And of course this fails if the patch has already maximum size.

Cc: <stable@vger.kernel.org> [3.3+]
Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/r/1344361461-10076-1-git-send-email-bp@amd64.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>