mac80211: add function to create data frame template including key

For some TDLS channel switch implementations data frames need to be
sent by the firmware based on a template. This template should be
created by mac80211, and thus needs to properly be built from an
802.3 frame into an 802.11 frame. In addition, the device will need
the key information so the select_key handler needs to be run.
However, the driver/device will be responsible for all of the crypto
encapsulation, as the sequence numbers etc. cannot be built by the
host anyway in this case since it's a template to be used multiple
times.

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