mac80211: introduce TDLS channel switch ops

Implement the cfg80211 TDLS channel switch ops and introduce new mac80211
ones for low-level drivers.
Verify low-level driver support for the new ops when using the relevant
wiphy feature bit. Also verify the peer supports channel switching before
passing the command down.

Add a new STA flag to track the off-channel state with the TDLS peer and
make sure to cancel the channel-switch if the peer STA is unexpectedly
removed.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>