ath9k: remove cache of rate preference when using 11g protection

No need to cache when we want to use 2Mbit/s for all protection
frames for 802.11g as we can determine that dynamically on
ath_buf_set_rate() itself.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>