iwlwifi: use ieee80211_conf to examine rate capabilities

This patch switches the use of internal iwlwifi structure with
ieee80211_conf in order to examine ht rate capabilities.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>