drm/i915: Fix documentation for __intel_wait_for_register_fw*

Use section name "Return" and proper error code -ETIMEDOUT

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190802124739.12548-1-michal.wajdeczko@intel.com