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

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83
Link: https://lore.kernel.org/r/20200619224334.GA7857@embeddedorSigned-off-by: Vinod Koul <vkoul@kernel.org>