xhci: remove unused 'ep_ring' variable in handle_cmd_completion()

This patch removes the variable 'ep_ring' that is assigned in
TRB_CONFIG_EP switch case but never used.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>