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Fix detection of unchanged chip contents on erase failure.
Return error if no usable erase functions exist.

Thanks to Stefan Tauner for spotting the last problem.

Corresponding to flashrom svn r1380.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>