dmaengine: Use unique debugfs names

DMA40 registers multiple dma_device instances for the same platform
device. The DMAengine debugfs code uses dev_name(dma_dev->dev) as the
directory name, so the second and third registrations try to create the
same directory and print duplicate-name warnings.

Keep the existing device-name directory for the first registration, but
use a dev_id-suffixed fallback when that name already exists.

Assisted-by: LLM
Signed-off-by: Linus Walleij <linusw@kernel.org>