i2c: mux: pca954x: Use dev_fwnode()

irq_domain_create_simple() takes fwnode as the first argument. It can be
extracted from the struct device using dev_fwnode() helper instead of
using of_node with of_fwnode_handle(). So use the dev_fwnode() helper.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
[wsa: proper commit header]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>