watchdog: tegra: Stop watchdog first if restarting

If we need to restart the watchdog due to someone changing the timeout
interval, stop the watchdog before restarting it.  Otherwise, the new
timeout doesn't seem to take.

Signed-off-by: Andrew Chew <achew@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>