watchdog: txx9wdt: fix timeout

timeout should be an unsigned int.
Set the timeout value properly in the watchdog_device struct so that
we don't get an faulty values for the WDIOC_GETTIMEOUT ioctl call.
Add check to see that timeout is a valid parameter after it is loaded
as a module.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>