watchdog: ath79_wdt: fix a typo in the name of a function

It is likely that 'ath97_wdt_shutdown()' should be 'ath79_wdt_shutdown()'

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>