dmaengine: dmatest: Replace symbolic permissions by octal permissions

Octal permissions are easier to read.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/a745b883288f95e999b71fac677bbc2daa13c22d.1654702928.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>