watchdog: iTCO_wdt: Use device managed resources

Using device managed resources simplifies error handling and cleanup,
and to reduce the likelyhood of errors.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>