usb: dwc2: use struct_size() in kzalloc()

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 foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo); instance = kzalloc(size, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

Notice that, in this case, variable size is not necessary, hence it is removed.

This code was detected with the help of Coccinelle.

---
 drivers/usb/dwc2/hcd.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

index dd82fa516f3f..3f087962f498 100644
@@ -3981,10 +3981,8 @@ static struct dwc2_hcd_urb *dwc2_hcd_urb_alloc(struct dwc2_hsotg *hsotg,
 					       gfp_t mem_flags)
 {
 	struct dwc2_hcd_urb *urb;
-	u32 size = sizeof(*urb) + iso_desc_count *
-		   sizeof(struct dwc2_hcd_iso_packet_desc);
 
-	urb = kzalloc(size, mem_flags);
+	urb = kzalloc(struct_size(urb, iso_descs, iso_desc_count), mem_flags);
 	if (urb)
 		urb->packet_count = iso_desc_count;
 	return urb;