PCI: mediatek-gen3: Rename mtk_pcie_port to mtk_gen3_pcie

Rename struct mtk_pcie_port to mtk_gen3_pcie to match the convention of
<driver>_pcie. No functional change intended.

Link: https://lore.kernel.org/r/20211223011054.1227810-15-helgaas@kernel.orgSigned-off-by: Fan Fei <ffclaire1224@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ryder Lee <ryder.lee@mediatek.com>
Cc: Jianjun Wang <jianjun.wang@mediatek.com>