ath9k: Rename ath_setcurmode() to ath_cache_conf_rate()

ath_setcurmode() is a bit misleading, all we are doing is
caching the rate for the corresponding configuration we
are using.

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