PCI: mediatek: Rename port->index and mtk_pcie_parse_ports()

Rename "port->index" to "port->slot" since the ports are hardwired at
PCI_SLOT.  Also rename "mtk_pcie_parse_ports()" to "mtk_pcie_parse_port()"
since it parses one port each time.

No functional change in this patch.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>