ath79: fix irq assignment for pci-ar71xx driver

ath79: fix irq assignment for pci-ar71xx driver

In the new DT-based pci-ar71xx driver, ar71xx_pci_irq_init() was being
called before populating the PCI controller's device_node struct member.
This led to no IRQ being assigned to connected PCI devices (e.g. ath9k
cards) and caused them to be non-functional aside from simply being
detected.

The previous errors encountered in dmesg were: "irq: no irq domain found
for /ahb/apb/pcie-controller@180c0000 !". /proc/interrupts listed an IRQ
of 0 for the cards.

While this has been only been tested on a yet-to-be-merged RouterStation
Pro target, it should also fix the broken wifi people have reported for
the ath79 WNDR3800 target.

Signed-off-by: Matt Merhar <mattmerhar@protonmail.com>