mac80211: add device_timestamp to the ieee80211_channel_switch struct

Some devices may need the device timestamp in order to synchronize the
channel switch.  To pass this value back to the driver, add it to the
channel switch structure and copy the device_timestamp value received
in the rx info structure into it.

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