usb: xhci-mtk: support to build xhci-mtk-hcd.ko

Currently xhci-hcd.ko building depends on USB_XHCI_MTK, this
is not flexible for some cases. For example:
USB_XHCI_HCD is y, and USB_XHCI_MTK is m, then we can't
implement extended functions if only update xhci-mtk.ko
This patch is used to remove the dependence.

Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/0b62e21ddfacc1c2874726dd27ccab80c993f303.1615170625.git.chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>