irqchip/econet-en751221: Switch to of_fwnode_handle()

of_node_to_fwnode() is an irqdomain's reimplementation of the
"officially" defined of_fwnode_handle(). The former is in the process of
being removed, so use the latter instead.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250513084739.2611747-1-jirislaby@kernel.org