ath6kl: Skip vif index validation in ath6kl_rx() for wmi events

When the wmi event is vif specific, the validation of vif index
is taken care in ath6kl_wmi_proc_events_iface(). This also avoids
the need for a netdev to be registered while receiving initial events
like "target_ready" and "regulatory domain".

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>