irqdomain: pci: Switch to of_fwnode_handle()

of_node_to_fwnode() is 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: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250319092951.37667-8-jirislaby@kernel.org