The channel_switch_beacon callback is optional, so it doesn't have to
be defined if it's not going to do anything useful with it.  Both
ath9k and ath10k define the callback and just returns.  This commit
removes them.

Cc: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: NLuciano Coelho <luciano.coelho@intel.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>