vrx518_ep: fix compilation error with kernel 6.12

vrx518_ep: fix compilation error with kernel 6.12

PCI_IRQ_LEGACY was renamed to PCI_IRQ_INTX for kernel 6.10. Fix
compilation error:

/home/db/owrt/build_dir/target-arm_cortex-a7+neon-vfpv4_musl_eabi/linux-ipq40xx_generic/vrx518_ep-2.1.0/ep.c:469:69: error: 'PCI_IRQ_LEGACY' undeclared (first use in this function); did you mean 'NR_IRQS_LEGACY'?
  469 |         err = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_MSI | PCI_IRQ_LEGACY);
      |                                                                     ^~~~~~~~~~~~~~
      |                                                                     NR_IRQS_LEGACY

Signed-off-by: Andre Heider <a.heider@gmail.com>
Co-authored-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>