ath5k: make use of the new rate control API

This patch enabels ath5k to use the new rate table to lookup each
mrr rate and retry information per packet.

Signed-off-by: Benjamin Vahl <bvahl@net.t-labs.tu-berlin.de>
Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>