dediprog: fix SPI clock setting.

Avoid setting SPI speed on firmware versions < 5.0.0 and note this
limitation in the man page.
Use the correct offset of the "12M" element in the spispeeds array to
match our manpage and the default of Dediprog's dpcmd.

Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://coreboot.org/flashrom/trunk@1674 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1