usb: define USB_SPEED_SUPER_PLUS speed for SuperSpeedPlus USB3.1 devices

BugLink: http://bugs.launchpad.net/bugs/1519623



Add a new USB_SPEED_SUPER_PLUS device speed, and make sure usb core can
handle the new speed.
In most cases the behaviour is the same as with USB_SPEED_SUPER SuperSpeed
devices. In a few places we add a "Plus" string to inform the user of the
new speed.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 8a1b2725)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>