phy: samsung: 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 samsung_usb2_phy_driver {
	...
        struct samsung_usb2_phy_instance instances[0];
};

instance = devm_kzalloc(dev, sizeof(struct samsung_usb2_phy_driver) +
			count * sizeof(struct samsung_usb2_phy_instance),
			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, instances, 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: Kishon Vijay Abraham I <kishon@ti.com>