ath9k: Exclude more functions

Use CONFIG_ATH9K_CHANNEL_CONTEXT to exclude more functions:

ath_offchannel_channel_change()
ath_scan_channel_start()
ath_scan_send_probe()

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