mac80211: add control port protocol TX control flag

A lot of drivers check the frame protocol for ETH_P_PAE,
for various reasons (like making those more reliable).
Add a new flags bitmap to the TX control info and a new
flag indicating the control port protocol is in use to
let all drivers also apply such logic to other control
port protocols, should they be configured.

Also use the new flag in the iwlwifi drivers.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>