dmaengine: hisilicon: Use struct_size() in devm_kzalloc()

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>
Reviewed-by: Zhou Wang <wangzhou1@hisilicon.com>
Link: https://lore.kernel.org/r/20200617211135.GA8660@embeddedor


Signed-off-by: Vinod Koul <vkoul@kernel.org>