programmer_delay: filter 0 usec delays.

We use 0 as delay value for some chips. Just skipping these here is the
most elegant, maintainable solution.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1760 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1