drm/i915/pxp: fix i915_pxp_tee_interface.h kernel-doc warnings

Make documentation match code. Slightly fix up the documentation
comments while at it.

v2:
- Move comments next to members instead of struct comment (Lucas)
- Small fixups while at it

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
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/7c26256dc00f970f94d145b73e341c36f553dfe4.1709898638.git.jani.nikula@intel.com