ath9k: Revert change that broke AR928X on Acer Ferrari One

Revert a hunk in drivers/net/wireless/ath/ath9k/hw.c introduced by
commit 2577c6e8

 ("ath9k_hw: Add support for AR946/8x chipsets") that
caused a nasty regression to appear on my Acer Ferrari One (the box
locks up entirely at random times after the wireless has been started
without any way to get debug information out of it).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>