We support this, so we should advertise it. In fact, if we don't,
mac80211 will do it for us (as we advertise SUPPORTS_VHT_EXT_NSS_BW
to it), but that requires a memory reallocation which is wasteful:

  ieee80211 phy0: copying sband (band 1) due to VHT EXT NSS BW flag

Set the flag here to avoid that.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>