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Some drivers support only a few combinations of read/write length and
return error otherwise. Having a distinct return code for this error
means we can handle it in upper layers.

Corresponding to flashrom svn r653.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Stefan Reinauer <stepan@coresystems.de>