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

[Minor commit message change -Mathias]
Signed-off-by: NCorentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: NMathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>