ath9k: Fix offchannel queuing

Clearing IEEE80211_TX_CTL_PS_RESPONSE in a frame
that is not in the current context doesn't seem right.
Instead make sure that we don't add such frames
to the UAPSD queue by using a local variable.

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