USB: Remove bogus USB_PORT_STAT_SUPER_SPEED symbol.

USB_PORT_STAT_SUPER_SPEED is a made up symbol that the USB core used to
track whether USB ports had a SuperSpeed device attached.  This is a
linux-internal symbol that was used when SuperSpeed and non-SuperSpeed
devices would show up under the same xHCI roothub.  This particular
port status is never returned by external USB 3.0 hubs.  (Instead they
have a USB_PORT_STAT_SPEED_5GBPS that uses a completely different speed
mask.)

Now that the xHCI driver registers two roothubs, USB 3.0 devices will only
show up under USB 3.0 hubs.  Rip out USB_PORT_STAT_SUPER_SPEED and replace
it with calls to hub_is_superspeed().

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>