A watchdog that can't be stopped returns -ENOSYS on set_timeout(0).
If the watchdog supports communicating whether it's running, we could
still allow `wd 0`, if it hasn't been started yet.

Setting device parameter .priority=0 disables a watchdog. One would
expect this to always succeed for a not-running watchdog, but currently
it doesn't, if the driver's set_timeout(0) returns -ENOSYS. With this
fix, this is supported making the user API less surprising.

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