watchdog: xen: use time64_t for timeouts

The Xen watchdog driver uses __kernel_time_t and ktime_to_timespec()
internally for managing its timeouts. Both are deprecated because of
y2038 problems. The driver itself is fine, since it only uses monotonic
times, but converting it to use ktime_get_seconds() avoids the deprecated
interfaces and is slightly simpler.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>