NAND: Davinci driver updates

Here comes a trivial patch to cpu/arm926ejs/davinci/nand.c. Unfortunately I
don't have hardware handy so I can not test it at the moment but changes are
rather trivial so it should work. It would be nice if somebody with a
hardware checked it anyways.

Signed-off-by: Sergey Kubushyn <ksi@koi8.net>