wifi: mac80211: set station RX-NSS on reconfig

When a station is added/reconfigured by userspace, e.g. a TDLS
peer or a SoftAP client STA, rx_nss is currently not always set,
so that it might be left zero. Set it up properly.

Link: https://msgid.link/20240129155354.98f148a3d654.I193a02155f557ea54dc9d0232da66cf96734119a@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>