ath5k: Fix short and long retry configuration

The register definition for retry configuration on AR5212 was wrong, and simply
copied over from AR5210. Update the register definitions from the documentation.

Let the short and long retries be configured from mac80211 and use the standard
values of 7 and 4 by default. Also we need to make sure we don't export more
retries than we are configured for to mac80211 (and the rate module) in
hw->max_rate_tries.

Also clean up the code by removing unused defines and variables and drop the
different values for "station retries" - if these need to be different it can
be handled tru ah_retry_long/short.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>