ath9k: fix check for antenna diversity support

fixes a regression on single-stream chips introduced in
commit 43c35284


"ath9k: implement .get_antenna and .set_antenna"

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>