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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>