iwlwifi: mvm: set correct chains in Rx status

ieee80211_rx_status::chains was always set to zero.
That caused rate scaling to always start with the
lowest rate possible (rs_get_initial_rate).
Set it correctly according to the MPDU response.

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>