ath9k_htc: Protect RX stream variables

Use a spin lock to prevent concurrent access
to variables dealing with RX stream mode handling.
Currently, no protection is implemented - which
causes problems in RX.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>