iwlwifi: mvm: add a channel_switch op to bypass mac80211 timer

We need to call ieee80211_chswitch_done() ourselves just when the
absence TE started, so we perform the actual context switch early
enough.  To do so, add a dummy channel_switch op, which will cause
mac80211 to skip the countdown timer and allow us to call
ieee80211_chswitch_done() to complete the operation.

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