watchdog: introduce watchdog_worker_should_ping helper

This will be useful when the condition becomes slightly more
complicated in the next patch.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Esben Haabendal <esben@haabendal.dk>
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>