mac80211: fix csa_counter_offs argument name in docbook

The csa_counter_offs was erroneously described as csa_offs in
the docbook section.

This fixes two warnings when making htmldocs (at least):

Warning(include/net/mac80211.h:3428): No description found for parameter 'csa_counter_offs[IEEE80211_MAX_CSA_COUNTERS_NUM]'
Warning(include/net/mac80211.h:3428): Excess struct/union/enum/typedef member 'csa_offs' description in 'ieee80211_mutable_offsets'

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