It's a parameter passed into the XHCI driver module
At trunk/linux-3.4.x/drivers/usb/host/xhci.c line 49:
int usb3_disable = 0;
module_param(usb3_disable, int, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(usb3_disable, "Disable USB3 interface");
And it disables USB3.0 port power if usb3_disable is set
At trunk/linux-3.4.x/drivers/usb/host/xhci-hub.c line 838:
if (usb3_disable && hcd->speed == HCD_USB3)
xhci_writel(xhci, temp & ~PORT_POWER, port_array[wIndex]);
else
xhci_writel(xhci, temp | PORT_POWER, port_array[wIndex]);