dmaengine: at_xdmac: 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 or integer overflows that, in
the worst scenario, could lead to heap overflows.

Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211208001013.GA62330@embeddedor


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