ath9k: improve max rate retry handling

ath9k currently forces hw->max_rate_tries to 4 to work around rate
control inefficiencies. This has some negative side effects, such as
rate_control_send_low also using a maximum of 4 tries, which could
negatively affect reliability of unicast management frames.
This patch pushes the retry limit to the rate control instead, and
allows it to use more tries on the last stage to prevent unnecessary
packet loss.

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