PCI: mediatek: Use PCI_NUM_INTX

Switch from using custom INTX_NUM macro to the generic PCI_NUM_INTX definition
for the number of INTx interrupts.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: use subject/changelog from similar patches]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>