ath9k: Fix channel context variables in ath_softc

chanctx_work and next_chan are required only when
CONFIG_ATH9K_CHANNEL_CONTEXT is enabled.

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