mac80211: add post_channel_switch driver operation

As a counterpart to the pre_channel_switch operation, add a
post_channel_switch operation.  This allows the drivers to go back to
a normal configuration after the channel switch is completed.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>