sb/intel/common/firmware: Reword me_cleaner warning

sb/intel/common/firmware: Reword me_cleaner warning

That vendor firmware still works after applying `me_cleaner` doesn't
mean that coreboot will also work with the same broken ME firmware.
Instead, one should first test coreboot with the original, unmodified ME
firmware to make sure coreboot works properly, and only then consider
using `me_cleaner` with coreboot. Otherwise, one would end up with a
non-booting or otherwise misbehaving system when trying to use coreboot
and `me_cleaner` without having tested coreboot with the original ME
firmware beforehand, which is hard to diagnose as the problem may only
happen when the ME isn't running normally.

Change-Id: I1626d747a99969faf7db37c10cf7d87e3977744a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60942
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>