libsa: smbios: probe: BCD revision parsing is v2-only code; Expand comments

The code parsing the BCD revision is only meaningful on v2, so move it
away into the appropriate 'if' branch to ease reading (and to avoid
a useless test).

Expand comments. In particular, make it clear that setting
'smbios.count' to '-1' removes the limit of the number of structures to
parse.

No functional change.

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49284

(cherry picked from commit d1f351fcb22f46805ff0a0cecf6ce787f84d31d5)