iwlwifi: mvm: use new pre_channel_switch op instead of channel_switch_beacon

A new callback has been added to prepare the device for a channel
switch.  Use the new callback instead of the old channel_switch_beacon
operation.

This makes it possible to remove the channel_switch_beacon operation
from mac80211.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>