watchdog: document watchdog_init_timeout() wdd parameter

All parameters of watchdog_init_timeout() are documented with exception
of wdd, thus generating a build warning.

This patch document it and so remove the following build warning:
drivers/watchdog/watchdog_core.c:113: warning: No description found for parameter 'wdd'
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
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>

drivers/watchdog/watchdog_core.c

@@ -97,6 +97,7 @@ static void watchdog_check_min_max_timeout(struct watchdog_device *wdd)
 
 /**
  * watchdog_init_timeout() - initialize the timeout field
+ * @wdd: watchdog device
  * @timeout_parm: timeout module parameter
  * @dev: Device that stores the timeout-sec property
  *