mtd: nand: lpc32xx_slc: fix calculation of timing arcs from given values

According to LPC32xx User's Manual all values measured in clock cycles
are programmable from 1 to 16 clocks (4 bits) starting from 0 in
bitfield, the current version of calculated clock cycles is too
conservative.

Correctness of 0 bitfield value (i.e. programmed 1 clock
timing) is proven with actual NAND chip devices.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>