[xhci] Set link state to RxDetect after disabling USB3 root hub port

[xhci] Set link state to RxDetect after disabling USB3 root hub port

The "disabled" port states for USB2 and USB3 are not directly
equivalent.  In particular, a disabled USB3 port will not detect new
device connections.  The result is that a USB3 device disconnected
from and reconnected to an xHCI root hub port will end up reconnecting
as a USB2 device.

Fix by setting the link state to RxDetect after disabling the port, as
is already done during initialisation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>