ath9k_hw: toggle weak signal detection in AP mode on older chipsets

The commit 80b4205b "ath9k: Fix OFDM weak signal detection for AP mode"
prevented weak signal detection changes from taking effect in AP mode on
all chipsets, claiming it is "not allowed".

The main reason for not disabling weak signal detection in AP mode is
that typically beacon RSSI is used to track whether it is needed to
boost range, and this is unavailable in AP mode for obvious reasons.

The problem with not disabling weak signal detection is that older
chipsets are very sensitive to high PHY error counts. When faced with
heavy noise, this can lead to an excessive amount of "Failed to stop
TX DMA" errors in the field.

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