iwlwifi: iwlmvm: in monitor NDP notif take the NSS from rx_vec

Take the NSS value from 'rx_vec' rather than from 'rate_n_flags'.
The rate_n_flags has only 2 bits for the NSS giving a max of 4SS
(0 = 1SS etc.). Since there may be up to 8SS use the rx_vec which
has 3 bits for the NSS.
While at it, fix the rx_vec array to length of 2.

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