xhci: remove unused hcd_index()

Now that each root hub has their own bus_state strucure the
hcd_undex() used to get the correct bus_state strucure is
no longer needed.

No functional changes

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>