dmaengine: ste_dma40_ll: Replace meaningless register set with comment

Unsure of the author's intentions, rather than just removing the nop,
we're replacing it with a comment containing the possible intention
of the statement OR:ing with 0.

Cc: Dan Williams <djbw@fb.com>
Cc: Per Forlin <per.forlin@stericsson.com>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>