PCI: mediatek: Configure FC and FTS for functions other than 0

PCI: mediatek: Configure FC and FTS for functions other than 0

"PCI_FUNC(port->slot << 3)" is always 0, so previously
mtk_pcie_startup_port() only configured FC credits and FTs for function
0.

Compute "func" correctly so we also configure functions other than
0. This affects MT2701 and MT7623.

Link: https://lore.kernel.org/r/c529dbfc066f4bda9b87edbdbf771f207e69b84e.1604510053.git.ryder.lee@mediatek.com
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>