mac80211: fix A-MPDU queue assignment

Internally, mac80211 requires the skb's queue mapping to be set
to the AC queue, not the virtual A-MPDU queue. This is not done
correctly currently, this patch moves the code down to directly
before the driver is invoked and adds a comment that it will be
moved into the driver later.

Since this requires __ieee80211_tx() to have the sta pointer,
make sure to provide it in ieee80211_tx_pending().

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>