pinctrl: mcp23s08: use struct_size() in devm_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;
    void *entry[];
};

instance = devm_kzalloc(dev, sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

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

instance = devm_kzalloc(dev, struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-mcp23s08.c

@@ -1139,8 +1139,7 @@ static int mcp23s08_probe(struct spi_device *spi)
 		return -ENODEV;
 
 	data = devm_kzalloc(&spi->dev,
-			    sizeof(*data) + chips * sizeof(struct mcp23s08),
-			    GFP_KERNEL);
+			    struct_size(data, chip, chips), GFP_KERNEL);
 	if (!data)
 		return -ENOMEM;