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>