zd1211rw: Fix TX status reporting in order to have proper rate control

First, we reduce the number of hardware retries to 0 (ie 2 real retries
for each rate). Next, when we report the retries to mac80211, we always
report a retry count of 1 (it seems to be 2 in fact, but using 2 seems
to lead to wrong performance for some reason). We use a state machine to
determine the real fate of a packet based on the 802.11 ACK and what the
Zydas hardware is saying when a real retry occurs. The real retry rates
are encoded in a static array. It has been tested with both zd1211 and
zd1211b hardware. Of course, since the Zydas hardware is not reporting
retries accurately, we are just doing our best in order to get the best
performance (ie higher throughput).

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>