dmaengine: ste_dma40: fix typo in enum documentation

s/40_command/d40_command to fix the below warning reported:

drivers/dma/ste_dma40.c:151: warning: expecting prototype for enum 40_command.
Prototype was for enum d40_command instead

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20230517064434.141091-2-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>