usb: xhci: remove unused variable ep

Fix the build warning: variable 'ep' set but not used

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