Input: applespi - 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. For example:

struct touchpad_protocol {
	...
        struct tp_finger        fingers[0];
};

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(*tp) + tp->number_of_fingers * sizeof(tp->fingers[0]);

with:

struct_size(tp, fingers, tp->number_of_fingers)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/applespi.c

@@ -1494,8 +1494,7 @@ static void applespi_got_data(struct applespi_data *applespi)
 		size_t tp_len;
 
 		tp = &message->touchpad;
-		tp_len = sizeof(*tp) +
-			 tp->number_of_fingers * sizeof(tp->fingers[0]);
+		tp_len = struct_size(tp, fingers, tp->number_of_fingers);
 
 		if (le16_to_cpu(message->length) + 2 != tp_len) {
 			dev_warn_ratelimited(&applespi->spi->dev,