Hopefully this will help anyone who unintentionally disables both USB configuration settings (XHCI controller, and USB 2.0 (EHCI) controller.
A friend of mine accidentally did this when trying to get his Asus T100TA (running Windows 8.1) to boot from a USB stick.

Once these USB settings are disabled the dock will no longer work. So you cannot get back into the UEFI BIOS using the Keyboard, to change the settings back.
I could not find a solution to this anywhere else, so figured out the following by trial and error.

Thanks to a useful forum on here I found out that by holding down the volume down button and restarting Windows 8, the tablet will boot to a startup menu which allows access to the UEFI setup menu (similar to the well known BIOS menu).

If the setup menu is not already highlighted, press the volume down button again until it is.
Then press the button just below the volume down button (aka "Windows" button), then the UEFI setup menu will appear.
Using the volume up and volume down buttons, and the windows button, you can then navigate to the USB settings and re-enable XHCI. The volume up button switches between the tabs in the UEFI BIOS setup menu, whereas volume down moves between the options in each tab. The windows button selects the setting or option highlighted.
After you have enabled XHCI, press the power button and volume down buttons simultaneously, to exit that menu and then navigate to the save changes and exit tab.
When the machine reboots you will have a working dock.