zd1211rw: 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(struct usb_req_write_regs) + count * sizeof(struct reg_data)

with:

struct_size(req, reg_writes, count)

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/zydas/zd1211rw/zd_usb.c

@@ -1917,8 +1917,7 @@ int zd_usb_iowrite16v_async(struct zd_usb *usb, const struct zd_ioreq16 *ioreqs,
 	if (!urb)
 		return -ENOMEM;
 
-	req_len = sizeof(struct usb_req_write_regs) +
-		  count * sizeof(struct reg_data);
+	req_len = struct_size(req, reg_writes, count);
 	req = kmalloc(req_len, GFP_KERNEL);
 	if (!req) {
 		r = -ENOMEM;