ath9k: Fix build error

This happens when CONFIG_ATH9K_CHANNEL_CONTEXT is
not enabled.

../drivers/net/wireless/ath/ath9k/recv.c: In function ‘ath_rx_ps_beacon’:
../drivers/net/wireless/ath/ath9k/recv.c:553:27: error: ‘struct ath_softc’ has no member named ‘offchannel’
    if (sc->cur_chan == &sc->offchannel.chan)
                           ^
../scripts/Makefile.build:257: recipe for target 'drivers/net/wireless/ath/ath9k/recv.o' failed
make[10]: *** [drivers/net/wireless/ath/ath9k/recv.o] Error 1
../scripts/Makefile.build:404: recipe for target 'drivers/net/wireless/ath/ath9k' failed
make[9]: *** [drivers/net/wireless/ath/ath9k] Error 2

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