dmaengine: ste_dma40: Use the BIT macro to replace ugly '(1 << x)'s

The aim is to make the code that little more readable.

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