Soon mac80211 will support multi-interface CSA so
using sc->csa_vif is not an option.

Instead just depend on vif->csa_active. Calling
ieee80211_csa_finish() multiple number of times
should not be an issue.
Signed-off-by: NMichal Kazior <michal.kazior@tieto.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>