watchdog: sp805: Set watchdog_device->timeout from ->set_timeout()

Implementation of ->set_timeout() is supposed to set 'timeout' field of 'struct
watchdog_device' passed to it. sp805 was rather setting this in a local
variable. Fix it.

Reported-by: Arun Ramamurthy <arun.ramamurthy@broadcom.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: <stable@vger.kernel.org> # 2.6.36+