ath9k: Fix OFDM weak signal detection for AP mode

The commit "ath9k_hw: improve ANI processing and rx desensitizing parameters"
changed the OFDM weak signal detection logic to disable it
for AP mode, which is not allowed. Fix this and enable it always
for AP mode.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>