usb: xhci: remove unused variable ep_ring

Fix the build warning about variable 'ep_ring' set but not used

[Minor commit message change -Mathias]
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>