usb: host: xhci-mtk: remove custom USB PHY handling

The new PHY wrapper is now wired up in the core HCD code. This means
that PHYs are now controlled (initialized, enabled, disabled, exited)
without requiring any host-driver specific code.
Remove the custom USB PHY handling from the xhci-mtk driver as the core
HCD code now handles this.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>