The stm32_iwdg watchdog can't be disabled. To have the wd commant report
this fact correctly to the user, the ->set_timeout needs to return -ENOSYS
which is interpreted as "Watchdog cannot be disabled" instead of -EINVAL
which means "Timeout value out of range".

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>