pxa3xx: fix ns2cycle equation

Test on a PXA310 platform with Samsung K9F2G08X0B NAND flash,
with tCH=5 and clk is 156MHz, ns2cycle(5, 156000000) returns -1.

ns2cycle returns negtive value will break NDTR0_tXX macros.

After checking the commit log, I found the problem is introduced by
commit 5b0d4d7c


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

To get num of clock cycles, we use below equation:
num of clock cycles = time (ns) / one clock cycle (ns) + 1
We need to add 1 cycle here because integer division will truncate the result.
It is possible the developers set the Min values in SPEC for timing settings.
Thus the truncate may cause problem, and it is safe to add an extra cycle here.

The various fields in NDTR{01} are in units of clock ticks minus one,
thus we should subtract 1 cycle then.

Thus the correct equation should be:
num of clock cycles = time (ns) / one clock cycle (ns) + 1 - 1
                    = time (ns) / one clock cycle (ns)

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Lei Wen <leiwen@marvell.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org