staging: greybus: power_supply: use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace code of the following form:

sizeof(*resp) + props_count * sizeof(struct gb_power_supply_props_desc)

with:

struct_size(resp, props, props_count)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/power_supply.c

@@ -520,8 +520,8 @@ static int gb_power_supply_prop_descriptors_get(struct gb_power_supply *gbpsy)
 
 	op = gb_operation_create(connection,
 				 GB_POWER_SUPPLY_TYPE_GET_PROP_DESCRIPTORS,
-				 sizeof(*req), sizeof(*resp) + props_count *
-				 sizeof(struct gb_power_supply_props_desc),
+				 sizeof(*req),
+				 struct_size(resp, props, props_count),
 				 GFP_KERNEL);
 	if (!op)
 		return -ENOMEM;