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>

drivers/mtd/nand/lpc32xx_slc.c

@@ -95,7 +95,7 @@
 * slc_tac register definitions
 **********************************************************************/
 /* Computation of clock cycles on basis of controller and device clock rates */
-#define SLCTAC_CLOCKS(c, n, s)	(min_t(u32, 1 + (c / n), 0xF) << s)
+#define SLCTAC_CLOCKS(c, n, s)	(min_t(u32, DIV_ROUND_UP(c, n) - 1, 0xF) << s)
 
 /* Clock setting for RDY write sample wait time in 2*n clocks */
 #define SLCTAC_WDR(n)		(((n) & 0xF) << 28)