ath9k_hw: fix 5 GHz frequency selection on AR934x/AR955x with 25 MHz refclock

The old code was an accidental copy&paste of the 2.4 GHz version,
which doesn't work.

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