platform/x86: i2c-multi-instantiate: 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.

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/i2c-multi-instantiate.c

@@ -81,9 +81,7 @@ static int i2c_multi_inst_probe(struct platform_device *pdev)
 	if (ret < 0)
 		return ret;
 
-	multi = devm_kmalloc(dev,
-			offsetof(struct i2c_multi_inst_data, clients[ret]),
-			GFP_KERNEL);
+	multi = devm_kmalloc(dev, struct_size(multi, clients, ret), GFP_KERNEL);
 	if (!multi)
 		return -ENOMEM;