dmaengine: stm32-mdma: Use struct_size() in kzalloc()

Make use of the new struct_size() helper instead of the offsetof() idiom.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201008141828.GA20325@embeddedor


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