mac80211: Fix mac80211.h docbook comments

A couple of enums in mac80211.h became structures recently, but the
comments didn't follow suit, leading to errors like:

  Error(.//include/net/mac80211.h:367): Cannot parse enum!
  Documentation/DocBook/Makefile:93: recipe for target 'Documentation/DocBook/80211.xml' failed
  make[1]: *** [Documentation/DocBook/80211.xml] Error 1
  Makefile:1361: recipe for target 'mandocs' failed
  make: *** [mandocs] Error 2

Fix the comments comments accordingly.  Added a couple of other small
comment fixes while I was there to silence other recently-added docbook
warnings.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>