ath6kl: wmi: use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace code of the following form:

sizeof(*ev) + ev->num_neighbors * sizeof(struct wmi_neighbor_info)

with:

struct_size(ev, neighbor, ev->num_neighbors)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath6kl/wmi.c

@@ -1295,8 +1295,7 @@ static int ath6kl_wmi_neighbor_report_event_rx(struct wmi *wmi, u8 *datap,
 	if (len < sizeof(*ev))
 		return -EINVAL;
 	ev = (struct wmi_neighbor_report_event *) datap;
-	if (sizeof(*ev) + ev->num_neighbors * sizeof(struct wmi_neighbor_info)
-	    > len) {
+	if (struct_size(ev, neighbor, ev->num_neighbors) > len) {
 		ath6kl_dbg(ATH6KL_DBG_WMI,
 			   "truncated neighbor event (num=%d len=%d)\n",
 			   ev->num_neighbors, len);