iwlwifi: mvm: implement VHT extended NSS support in rs.c

iwlwifi: mvm: implement VHT extended NSS support in rs.c

For non-offloaded rate control, implement VHT extended NSS
support by just ignoring 160 MHz on transmit if the peer
doesn't support the full NSS (2). This is pretty unlikely
and gets us 160 MHz RX as well as TX in most cases, since
the typical case for this workaround is 4x4 AP only doing
2x2 on 160 MHz.

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