usb: xhci-mtk: Use struct_size() helper in create_sch_ep()

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 worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/usb/host/xhci-mtk-sch.c:265:20: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220120015546.GA75917@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-mtk-sch.c

@@ -248,7 +248,6 @@ create_sch_ep(struct xhci_hcd_mtk *mtk, struct usb_device *udev,
 	struct mu3h_sch_bw_info *bw_info;
 	struct mu3h_sch_tt *tt = NULL;
 	u32 len_bw_budget_table;
-	size_t mem_size;
 
 	bw_info = get_bw_info(mtk, udev, ep);
 	if (!bw_info)
@@ -262,9 +261,9 @@ create_sch_ep(struct xhci_hcd_mtk *mtk, struct usb_device *udev,
 	else
 		len_bw_budget_table = 1;
 
-	mem_size = sizeof(struct mu3h_sch_ep_info) +
-			len_bw_budget_table * sizeof(u32);
-	sch_ep = kzalloc(mem_size, GFP_KERNEL);
+	sch_ep = kzalloc(struct_size(sch_ep, bw_budget_table,
+				     len_bw_budget_table),
+			 GFP_KERNEL);
 	if (!sch_ep)
 		return ERR_PTR(-ENOMEM);