rtc: ds1374: wdt: Fix stop/start ioctl always returning -EINVAL

The WDIOC_SETOPTIONS case in the watchdog ioctl would alwayss falls
through to the -EINVAL case. This is wrong since thew watchdog does
actually get stopped or started correctly.

Fixes: 920f91e5 ("drivers/rtc/rtc-ds1374.c: add watchdog support")
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>