pinctrl: tb10x: Use flexible-array member and struct_size() helper

Update the code to use a flexible array member instead of a pointer in
structure tb10x_pinctrl and use the struct_size() helper:

struct tb10x_pinctrl {
        ...
	struct tb10x_of_pinfunc pinfuncs[];
};

Also, make use of the struct_size() helper instead of an open-coded
version in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(struct tb10x_pinctrl) + of_get_child_count(of_node) * sizeof(struct tb10x_of_pinfunc)

with:

struct_size(state, pinfuncs, of_get_child_count(of_node))

This code was detected with the help of Coccinelle.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>