Park masked U3 ports instead of leaving them half-enabled

[meehien]

xhci_mtk_host_enable() skips ports listed in mediatek,u3p-dis-msk
entirely — it neither powers them up nor down, so they keep the bootloader
default, which on MediaTek IPPC is powered up and host-selected. But the
mask exists precisely for ports whose U3 PHY is unusable: on the R4 Pro,
ssusb0's U3 lane is muxed to PCIe2, so the DT masks U3 port 0 and omits the
PHY.

The port is therefore powered and host-selected with no initialized PHY and
a MAC clock domain never released from reset, holding the controller's level
interrupt asserted with a status xhci_irq() cannot acknowledge. The
spurious-IRQ detector disables the line, and the perfectly good USB2 bus on
the same controller dies with it — completions never arrive, the first
hub_event needing a command parks in D state holding the hub lock, and
device_shutdown() blocks behind it, hanging reboot.

Explicitly set CTRL_U3_PORT_PDN and CTRL_U3_PORT_DIS for masked ports,
mirroring what host_enable() does in reverse for active ones.

Companion to the clk-mt7988 fix (PR 1) — same board, same symptom class,
independent causes. Both are needed.