dmi scan: warn about too early calls to dmi_check_system()

It happened to me recently that i added a dmi_check_system() quirk
in a too early codepath, and it was silently ignored because all the
DMI tables and strings were still empty.

As this situation is clearly a programming error / kernel bug,
warn when it happens, instead of silently ignoring quirks.

Signed-off-by: Ingo Molnar <mingo@elte.hu>