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