watchdog: mena21_wdt: Fix possible NULL pointer dereference

In a21_wdt_remove() we do a watchdog_unregister_device() on struct
a21_wdt_drv->wdt but never assign it.

Also move the dev_set_drvdata() call in front of the watchdog_register_device()
call, so it doesn't look like an error.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>