ucode: Fix validation on Intel platforms

The check for the extended signature table was backwards, so we always
ignored it.

We should verify that the extended signature table fits within the total
image size.

Reviewed by: jrm, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D57209

(cherry picked from commit 0beb172898499fff51eed4df3d9284cd1094afbb)