ath9k_htc: Fix memory leak on WMI event handler

ath9k_wmi_ctrl_rx is racy with ath9k_wmi_tasklet on event notification
due to which the wmi_skb may be overwritten which leads to memory leak.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>