watchdog: Fix null pointer dereference while accessing rdc321x platform_data

rdc321x-wdt currently fetches its driver specific data by using the
platform_device->platform_data pointer, this is wrong because the mfd
device which registers our platform_device has been added using
mfd_add_device() which sets the platform_device->driver_data pointer
instead.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
CC: stable@kernel.org
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>