zd1211rw: zd_usb: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct usb_int_regs {
	...
        struct reg_data regs[0];
} __packed;

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following function:

static int usb_int_regs_length(unsigned int count)
{
       return sizeof(struct usb_int_regs) + count * sizeof(struct reg_data);
}

with:

struct_size(regs, regs, 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

@@ -1597,11 +1597,6 @@ static int zd_ep_regs_out_msg(struct usb_device *udev, void *data, int len,
 	}
 }
 
-static int usb_int_regs_length(unsigned int count)
-{
-	return sizeof(struct usb_int_regs) + count * sizeof(struct reg_data);
-}
-
 static void prepare_read_regs_int(struct zd_usb *usb,
 				  struct usb_req_read_regs *req,
 				  unsigned int count)
@@ -1636,10 +1631,10 @@ static bool check_read_regs(struct zd_usb *usb, struct usb_req_read_regs *req,
 	/* The created block size seems to be larger than expected.
 	 * However results appear to be correct.
 	 */
-	if (rr->length < usb_int_regs_length(count)) {
+	if (rr->length < struct_size(regs, regs, count)) {
 		dev_dbg_f(zd_usb_dev(usb),
-			 "error: actual length %d less than expected %d\n",
-			 rr->length, usb_int_regs_length(count));
+			 "error: actual length %d less than expected %ld\n",
+			 rr->length, struct_size(regs, regs, count));
 		return false;
 	}