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This does not only remove a huge pile of duplicate code, it does
also fix a bug in spi_disable_blockprotect_at25df(), which is also
a good example why duplicated code is a bad idea.

Corresponding to flashrom svn r1635.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>