watchdog: softdog: use watchdog core to init timeout value

Error string and comment say we fall back to a default, but in reality
we bailed out. Refactor the code to use the core helper which then
matches the described behaviour. While updating the init message anyhow,
shorten it while we are here; no need for versioning there as well and
the name is already given via pr_fmt.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.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@iguana.be>