watchdog: ziirave_wdt: Fix misleading error message

Fix misleading error message in ziirave_wdt_init_duration(). Saying
"unable to set ..." implies that an attempt at communication with
watchdog device has taken palce and was not successful. In this case,
however, all it indicates is that no reset pulse duration was
specified either via kernel parameter or Device Tree. Re-phase the log
message to be more clear about benign nature of this event.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Rick Ramstetter <rick@anteaterllc.com>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190812200906.31344-16-andrew.smirnov@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>