i2c: mux: gpio: Don't dereference fwnode from struct device

We have a special helper to get fwnode out of struct device.
Moreover, dereferencing it directly prevents the fwnode
modifications in the future.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>