wifi: mac80211: add kernel-doc for EHT structure

Looks like this is required, even if all of the members
are separately described. Add a line to avoid the warning.

Fixes: f66c48af ("mac80211: support minimal EHT rate reporting on RX")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>