mac80211: use correct channel in TX

Since we only need the band, remove the channel
pointer from struct ieee80211_tx_data and also
assign it properly, depending on context, to the
correct operating or current channel.

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

diff --cc net/mac80211/ieee80211_i.h
Simple merge

diff --cc net/mac80211/tx.c
Simple merge