Constify dediprog's write functions too.

I forgot doing so in r1789 which broke compiling the dediprog module with
-Werror (which is default). Thanks to Mike Hibbett for reporting this.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1797 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1