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

[olce]

Summary

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.

[markj]

stand/libsa/smbios.c

586

"No." is a somewhat common abbreviation for "number". "Nb" looks like a typo to me. If you're going to change it, I'd rather use "No." or just "Number".