usb: host: ehci-sched: Use struct_size() in kzalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worse scenario, could lead to heap overflows.

Also, address the following sparse warning:
drivers/usb/host/ehci-sched.c:1168:40: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174
Acked-by: Alan Stern <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/20220111075427.GA76390@embeddedor
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/ehci-sched.c

···
 iso_sched_alloc(unsigned packets, gfp_t mem_flags)
 {
 	struct ehci_iso_sched	*iso_sched;
-	int			size = sizeof(*iso_sched);
 
-	size += packets * sizeof(struct ehci_iso_packet);
-	iso_sched = kzalloc(size, mem_flags);
+	iso_sched = kzalloc(struct_size(iso_sched, packet, packets), mem_flags);
 	if (likely(iso_sched != NULL))
 		INIT_LIST_HEAD(&iso_sched->td_list);