mac80211: remove redundant IEEE80211_STA_CSA_RECEIVED flag

The csa_active flag was added in sdata a while ago and made
IEEE80211_STA_CSA_RECEIVED redundant.  The new flag is also used to
mark when CSA is ongoing on other iftypes and took over the old one as
the preferred method for checking whether we're in the middle of a
channel switch.  Remove the old, redundant flag.

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