dmaengine: ste_dma40: Allocate plat_data on declaration

It's the way that most other drivers do it.

Very trivial clean-up which reduces line count and simplifies code.

Acked-by: Vinod Koul <vnod.koul@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>