watchdog: sama5d4: fix timeout-sec usage

When using watchdog_init_timeout to update the default timeout value,
an error means that there is no "timeout-sec" in the relevant device
tree node.

This should not prevent binding of the driver to the device.

Fixes: 976932e4 ("watchdog: sama5d4: make use of timeout-secs provided in devicetree")
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>
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>