dmaengine: ste_dma40: Fix up static memcpy event lines

The commit
"dmaengine: ste_dma40: Assign memcpy channels in the driver"
had two problems:
- It assigned magic numbers to the memcpy event lines
  instead of giving them symbolic names.
- It seems to have missed one of the memcpy event lines,
  number 51.

This patch fixes it up.

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