PCI: mediatek: Use bus->sysdata to get host private data

75983c6d1f38 ("PCI: mediatek: Add controller support for MT2712 and
MT7622") has put the mtk_pcie * into bus->sysdata.  Take advantage of that
to get the private data and simplify the code.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>