watchdog: coh901327_wdt.c: fix timeout

Set the timeout value properly so that we don't get faulty values
for the WDIOC_GETTIMEOUT iotcl. 'margin' should be an unsigned int.
Also add a check to see if margin is a valid parameter after it is
loaded as a module.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>