ath9k: Remove SC_OP_OFFCHANNEL

mac80211's IEEE80211_CONF_OFFCHANNEL can be used directly
to track offchannel state. Also pass the correct boolean
value to ath9k_hw_startpcureceive().

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