watchdog: mena21: Do not use device pointer from struct watchdog_device

The device pointer in struct watchdog_device has a different lifetime
than the driver code and should not be used in drivers. Use the pointer
to the parent device instead.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>