that hwirq corresponds to the second argument of function irq_linear_revmap:
(This line is taken from line 284 of arch/mips/pci/pci-ar71xx.c)
It's completely software defined here although it's called "hwirq". The "hwirq" means the corresponding hardware IRQ number of the current IRQ chip. In our case here, we don't actually have a hardware IRQ chip, we just read the interrupt status register and execute the corresponding IRQ handler of pci devices. We registered an IRQ chip in pci driver but that "IRQ chip" is dummy.
I suggest adding some debug print in ar71xx_pci_irq_handler and check which bit of AR71XX_RESET_REG_PCI_INT_STATUS is triggered when receving an interrupt.
I think I can't help further here due to lack of a router.