Fix error in timing generation, Tacls is only in the range 0..3

Add proper support for the s3c2440 NAND controller, which has now
been tested on several s3c2440 implementations.
Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>