[WATCHDOG] Stop looking for device as soon as one is found

If no address is given for the W83697HF/HG watchdog IO port, stop looping
through possible locations when a watchdog device has been found.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>