iwlwifi: filling Tx MCS set

This patch fills the needed data about HW capabilities in matters of
possible HT Tx MCS.

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