[WATCHDOG] iTCO_wdt: clean up probe(), modify err msg

It's possible that the platform is not allowing reboot via TCO timer
expiration.

Also, differentiate between not finding a chipset that has TCO, and the case
where TCO is present but the driver fails to initialize for some reason.

Signed-off-by: Naga Chumbalkar <nagananda.chumbalkar@hp.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>