usb: mtu3: add feature to disable device's usb3 port

We may want to disable device's usb3 port when the combo phy is switched
from usb3 mode to pcie mode for some projects.
Meanwhile rename member @is_u3_ip to @u3_capable to avoid misleading, due
to the member's value may be changed when disable usb3 port, but the
controller is still a usb3 IP which also tells us how to manage the fifo.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20220708071903.25752-3-chunfeng.yun@mediatek.com


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