power: supply: cros_peripheral: Use struct_size() helper in kzalloc()

Similar to the handling of cros_usbpd_charger_ec_command in commit 441d38c6
("power: supply: cros_usbpd: Use struct_size() helper in kzalloc()"),
we thought a patch might be needed here as well.

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Signed-off-by: Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>