iwlwifi: add channel switch support to 5000 series and up

Support "channel switch" request by issue "channel switch" host command
to uCode.

There is no separated "channel switch" indication from mac80211,
when detected "IEEE80211_CONF_CHANGE_CHANNEL" flag in iwl_mac_config(),
if the station is in "associated" state, then assume "channel switch
announcement" IE was received by mac80211.

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>