watchdog: mt7621: switch to using managed devm_watchdog_register_device()

This does the necessary cleanup on driver unload automatically.

Signed-off-by: André Draszik <git@andred.net>
Cc: linux-watchdog@vger.kernel.org
Cc: John Crispin <john@phrozen.org>
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>