ath9k: Fix channel context events

Check if channel context usage is enabled before
calling ath_chanctx_event() from various parts of the
driver. Also, make sure that ath_chanctx_event() is
compiled 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>