[WATCHDOG] max63xx: be careful when disabling the watchdog

When shutting down the watchdog timer, special care must be taken
not to overwrite other bits in the register, as it may be shared
with other peripherals.

For example, on the Arcom Vulcan, the register is shared between
the watchdog and the PCI reset line...

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>