DMI: allow omitting ident strings in DMI tables

The purpose of dmi->ident is twofold - it may be used by DMI callback
functions when composing log messages; it is also used to determine
end of DMI table in dmi_check_system() and dmi_first_match(). However,
in case when callbacks are not interested in using ident at all it just
wastes memory. Let's make entries with empty first match slot serve as
end-of-table markers instead.

Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>