mac80211: remove antenna_sel_tx TX info field

This field is never set to anything non-zero in
mac80211, so we should be able to remove it.
Unfortunately though, the iwlwifi and iwlegacy
drivers use it for their internal TX status
processing (which shouldn't be using the rate
control API to start with), so add a new field
"status.antenna" for them, at least for now.

In the future, I plan to use the new field to
hold the hardware queue, while the SKB's queue
mapping holds the AC.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>