ath9k_htc: add STBC TX support

All known ar7010+ar* device and current FW support STBC TX. This patch
make use of it and suggest to send STBC if peer support it.
I use wort "suggest" since currenly we have separate rate controller
in FW which will make decision based on rate and hardware.

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