mac80211: notify NSS changed when IBSS and HT

When using IBSS in HT mode, we always get NSS=1
in rc_update callback. Force NSS recalculation when
rates updated and notify driver that NSS changed.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>