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Add paranoid checks for correct values in essential registers in the SB600/SB700/... SPI driver. If something else changes the values we
wrote, we will see severe read/write corruption.
sb600spi will now abort the access and return an error if it detects
this sort of corruption.

Note: This corruption can be caused by a few different events:
- IPMI/BMC/IMC accesses flash
- Other software accesses flash
The nature of flash access (read/write/ID/...) is irrelevant. Each such
access will cause corruption for all other accesses happening at the
same time.

Thanks to Matthias Kretz for testing this patch.

Corresponding to flashrom svn r1145.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Matthias Kretz <kretz@kde.org>