ath9k: Isolate ath9k_use_chanctx module parameter

This patch ensures that the module parameter "use_chanctx" is
visible only when CONFIG_ATH9K_CHANNEL_CONTEXT is selected.
Also register the channel context callbacks with mac80211 only
when it is explicitly enabled and compile them out of the driver
when CONFIG_ATH9K_CHANNEL_CONTEXT is not selected.

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