i2c: core: Do not dereference fwnode in struct device

In order to make the underneath API easier to change in the future,
prevent users from dereferencing fwnode from struct device.
Instead, use the specific device_set_node() API for that.

Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>