hwspinlock: sprd: Use struct_size() helper in devm_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 worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/hwspinlock/sprd_hwspinlock.c:96:36: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Baolin Wang <[e-mail]>
Signed-off-by: Bjorn Andersson <[e-mail]>
Link: https://lore.kernel.org/r/20220125225723.GA78256@embeddedor

drivers/hwspinlock/sprd_hwspinlock.c

···
 		return -ENODEV;
 
 	sprd_hwlock = devm_kzalloc(&pdev->dev,
-				   sizeof(struct sprd_hwspinlock_dev) +
-				   SPRD_HWLOCKS_NUM * sizeof(*lock),
+				   struct_size(sprd_hwlock, bank.lock, SPRD_HWLOCKS_NUM),
 				   GFP_KERNEL);
 	if (!sprd_hwlock)
 		return -ENOMEM;