watchdog: ts72xx_wdt: disable watchdog at probe

Since it may be already enabled by bootloader or some other utility. This patch
makes sure that the watchdog is disabled before any userspace daemon opens the
device. It is also required by the watchdog API.

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>