ath9k: Conditionally compile more functions

Use CONFIG_ATH9K_CHANNEL_CONTEXT to exclude:

ath_scan_complete()
ath_roc_complete()
ath_offchannel_next()
ath_scan_next_channel()
ath_scan_channel_duration()

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