drm/i915: fix i915_gsc_proxy_mei_interface.h kernel-doc

There's no proper way to document function pointer members, but at least
silence the warnings.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a84bd76162290940f709f5cb6e432e5e1f75a3b9.1709898638.git.jani.nikula@intel.com