ath9k: Conditionally compile a few functions

Exclude these functions when CONFIG_ATH9K_CHANNEL_CONTEXT is not
enabled:

ath_chanctx_send_vif_ps_frame()
ath_chanctx_send_ps_frame()
ath_chanctx_defer_switch()
ath_chanctx_set_next()

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