xhci: set slot context speed field to SuperSpeedPlus for USB 3.1 SSP devices

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



The speed field of the input slot context should represent the speed the
device is working at.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit d7854041)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>