iio: adc: stm32-adc: Use dev_fwnode()

iio: adc: stm32-adc: 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>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250612084627.217341-1-jirislaby@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>