ath9k: ar9003_mac: read STBC indicator from rx descriptor

The rx descriptor contains a STBC indicator. If the indicator is set the
frame was received with STBC.

Signed-off-by: Philipp Borgers <borgers@mi.fu-berlin.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210517172426.15919-1-borgers@mi.fu-berlin.de