ath9k: Fix 'offchannel' in ath_softc

Finally move the 'offchannel' instance in ath_softc
inside a CONFIG_ATH9K_CHANNEL_CONTEXT cage. The offchannel
usage in ath9k_calculate_iter_data() is closed off with
an ifdef for now, since the state/opmode calculation is
common for both the channel context mode and the normal mode.

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