x86/ucode: add const where appropriate

Approved by: so
Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43865

(cherry picked from commit fd24a63a38d0d4cb4f5041ef232036abe09d9d4f)
(cherry picked from commit 1e857a99478b77b862380d5f64c127bde0f7c9ac)