iwlwifi: fix HT operation in 2.4 GHz band

When we cleaned up the driver to properly tell mac80211 about HT rates
("iwlwifi: use iwl_hwrate_get_mac80211_idx where appropriate"), we broke
internal rate indexing in 2.4 GHz band.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Tested-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>