[WATCHDOG] Use DIV_ROUND_UP() macro in the coh901327 WDT

I saw Julia Lawalls various commits fixing up the use of rounding
macros and since my already submitted patch was not caught in this
I took it upon myself to fix it up for this driver as well.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>