ath9k: mark wmi_event_swba as __packed

The other structures in wmi.h are already marked this way.
Without this marking, we get an unaliged access panic in the tilegx kernel:

Starting stack dump of tid 0, pid 0 (swapper) on cpu 35 at cycle 198675113844
frame 0: 0xfffffff7103ada90 ath9k_htc_swba+0x120/0x618 [ath9k_htc]
frame 1: 0xfffffff7103a4b10 ath9k_wmi_event_tasklet+0x1b0/0x270 [ath9k_htc]
frame 2: 0xfffffff700326570 tasklet_action+0x148/0x298
[...]

Signed-off-by: Chris Metcalf <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/ath/ath9k/wmi.h

···
 struct wmi_event_swba {
 	__be64 tsf;
 	u8 beacon_pending;
-};
+} __packed;
 
 /*
  * 64 - HTC header - WMI header - 1 / txstatus