ath9k: disable TPC support again (for now)

TPC support has been observed to cause some tx power fluctuations on
some devices with at least AR934x and AR938x chips.
Disable it for now until the bugs have been found and fixed

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>