[MTD] [NAND] pxa3xx: convert from ns to clock ticks more accurately

The various fields in NDTR{01} are in units of clock ticks minus one, but the
ns2cycle macro mistakenly adds one, inflating the number of clock ticks and
making it impossible to set any of these fields to zero.

Signed-off-by: Matt Reimer <mreimer@vpop.net>
Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>