PCI: mediatek-gen3: Do full device power down on removal

PCI: mediatek-gen3: Do full device power down on removal

When power control for downstream devices was introduced in the
mediatek-gen3 PCIe controller driver, only the power to the downstream
devices was cut when the controller driver is removed. This matched
existing behavior, but in hindsight a proper power down sequence should
have been followed.

Call mtk_pcie_devices_power_down() on driver removal so that in addition
to removing power from the downstream devices, PERST# is asserted.

Fixes: 1a152e21940a ("PCI: mediatek-gen3: Integrate new pwrctrl API")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260505105918.1823170-1-wenst@chromium.org