mac80211: fixme for kernel-doc

Fix kernel-doc warnings in mac80211.h.
Fields need real explanations added to them.

Warning(lin2627-g3-kdocfixes//include/net/mac80211.h:659): No description found for parameter 'icv_len'
Warning(lin2627-g3-kdocfixes//include/net/mac80211.h:659): No description found for parameter 'iv_len'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>