cfg80211: add documentation for max_num_csa_counters

Move the comment in the structure to a description of the
max_num_csa_counters field in the docbook area.

This fixes a warning when building htmldocs (at least):

 Warning(include/net/cfg80211.h:3064): No description found for parameter 'max_num_csa_counters'

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>