ath9k: check for Rx-STBC flag and pass it to ieee80211

This patch make use of STBC flag in DMA RX descriptor.
Only devices after ar9280 can provide this information.

If card support it we will set HAVE_STBC flag, to show
clint programm thet STBC is supported but not received.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>