drm/i915/pxp: fix kernel-doc for member dev_link

Add /** to make it a kernel-doc.

drivers/gpu/drm/i915/pxp/intel_pxp_types.h:96: warning: Function parameter or member 'dev_link' not described in 'intel_pxp'

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/84f89a3332fa323888f1e3241fb51ae10417ecd7.1683041799.git.jani.nikula@intel.com