cpucontrol: return selected revision from ucode_amd_find()

This fixes two printing bugs in cpucontrol(1). First, the utility will
now print "updating from rev X to rev Y", instead of incorrect "updating
to revision X", where X is actually the old revision. This also matches
what Intel updater prints. Second, the utility won't incorrectly warn
that the update failed after reading the new revision post update.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D52506

(cherry picked from commit 83804499b72405475027e670690d4cdbada46090)