firmware: dmi_scan: Drop dmi_initialized

I don't think it makes sense to check for a possible bad
initialization order at run time on every system when it is all
decided at build time.

A more efficient way to make sure developers do not introduce new
calls to dmi_check_system() too early in the initialization sequence
is to simply document the expected call order. That way, developers
have a chance to get it right immediately, without having to
test-boot their kernel, wonder why it does not work, and parse the
kernel logs for a warning message. And we get rid of the run-time
performance penalty as a nice side effect.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Ingo Molnar <mingo@kernel.org>