ath9k: Check multi-channel context for WOW

If CONFIG_ATH9K_CHANNEL_CONTEXT is enabled, check whether
multiple contexts are active and if so, return 1 without
enabling WOW since we don't support it in this case.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>