drm/i915/guc: Fix return from guc_log_relay_file_create

drm/i915/guc: Fix return from guc_log_relay_file_create

guc_log_relay_file_create will return -EEXIST if we invoke
relay_late_setup_files multiple times as part of i915_guc_log_control.
However this is to be not cosidered as fail and need to return 0.
This was mistakenly introduced in the below commit. Fix it.

Fixes: 70deeaddc6e6 "drm/i915/guc: Fix lockdep due to log relay channel handling under struct_mutex"
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1517379279-12967-1-git-send-email-sagar.a.kamble@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>