mac80211: refactor ieee80211_mesh_process_chanswitch()

Refactor ieee80211_mesh_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: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>