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We got enough (and no one is looking at them for the time being anyway).
Also, return an error code in the case no bus type could be detected.

Corresponding to flashrom svn r1820.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>