ath6kl: debug: 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, change the following form:

sizeof(*tbl) + num_entries * sizeof(struct wmi_bss_roam_info)

 to :

struct_size(tbl, info, num_entries)

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/debug.c

@@ -1132,8 +1132,7 @@ int ath6kl_debug_roam_tbl_event(struct ath6kl *ar, const void *buf,
 
 	tbl = (const struct wmi_target_roam_tbl *) buf;
 	num_entries = le16_to_cpu(tbl->num_entries);
-	if (sizeof(*tbl) + num_entries * sizeof(struct wmi_bss_roam_info) >
-	    len)
+	if (struct_size(tbl, info, num_entries) > len)
 		return -EINVAL;
 
 	if (ar->debug.roam_tbl == NULL ||