ath6kl: disallow WoW with multiple vifs

The ath6kl fw does not have enough memory to support the WoW filters of
more than one vif. Disallow WoW suspend if this is the case.

Signed-off-by: Thomas Pedersen <c_tpeder@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>