drm/dp: Fixup kernel docs for struct drm_dp_aux

* Make sure that struct members are referred to using @, otherwise they
  won't be formatted as such
* Make sure to refer to other struct types using & so they link back to
  each struct's definition
* Make sure to precede constant values with % so they're formatted
  correctly

Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326203807.105754-2-lyude@redhat.com