Airtime link metric estimation was broken in HT mesh, use
cfg80211_calculate_bitrate to get the right rate value.

Also factor out tx rate copying from sta_set_sinfo().
Signed-off-by: NThomas Pedersen <thomas@cozybit.com>
Reviewed-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>