iwlwifi: avoid race condition in channel change

When iwl_mac_config() is called by mac80211, the channel pointer
hw->conf->channel can potentially change, resulting in mismatch
band and channel number when configuring RXON command. To avoid
this situation, save the channel pointer in local variables
and validate the channel before using it. Note that priv->mutex
is locked during the whole function so the local variables are safe.

Same change is applied to iwl_mac_channel_switch() since basically
it copies code from iwl_mac_config().

Also removed an outdated comment in the flow.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>