wifi: cfg80211: add missing kernel-doc for cqm_rssi_work

As reported by Stephen, I neglected to add the kernel-doc
for the new struct member. Fix that.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 37c20b2e

 ("wifi: cfg80211: fix cqm_config access race")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>