ath5k: implement multi-rate retry support, fix tx status reporting

Clean up the tx status reporting, fix retry counters (short retries are
virtual collisions, not actual retries). Implement multi-rate retry
support.
This also fixes strong throughput fluctuations with rc80211_pid

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>