Refactor ieee80211_ibss_process_chanswitch() to use
ieee80211_channel_switch() and avoid code duplication.

Tested by: Simon Wunderlich <sw@simonwunderlich.de>
Acked by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: NLuciano Coelho <luciano.coelho@intel.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>