iwlwifi: properly set basic rates

This fixes a long-standing bug: iwlwifi always assumes
that the CCK ACK rates are 1 and 2 MBps and the OFDM
ACK rates are 6, 12 and 24 MBps. Fix this problem by
using the basic rates the AP (or in AP case hostapd)
told us to use and add the necessary mandatory rates
to the mix.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>