iwlwifi: add IEEE80211_AMPDU_TX_OPERATIONAL

mac80211 do not check the return code now, what if mac80211 does start
using the return code? IEEE80211_AMPDU_TX_OPERATIONAL is a valid action,
just iwlwifi driver do not need to take any action for it; so instead of
return "-EINVAL", it is a good program practice to return "-EOPNOTSUPP"
to make sure mac80211 will not get wrong impression.

Signed-off-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>