i2c: cht-wc: Replace of_node by NULL

The driver is run on the platforms where OF node is always NULL.
The confusion comes from IRQ domain APIs that take either OF or
firmware node as input parameter. Since fwnode is not used here
either, replace of_node by NULL.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>