tools/sdiodevs2h.awk: fix after removal of $FreeBSD$

The input file used to have a $FreeBSD$ tag on the first line and we
recorded that in the generated files to know which versions they were
based on. With the removal of $FreeBSD$ the logic was not quite
correct anymore; fix that to generate proper header files again.

Sponsored by: The FreeBSD Foundation
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D46300

(cherry picked from commit 499c337781bf8df7f97f612b93427b3f53dfd8bb)