mac80211: clean up __ieee80211_tx args

__ieee80211_tx takes a struct ieee80211_tx_data argument, but only
uses a few of its members, namely 'skb' and 'sta'. Make that explicit,
so that less internal knowledge is required in ieee80211_tx_pending
and the possibility of introducing errors here is removed.

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>