Channel device name is used for sysfs, but also by dmatest filter function.

With dynamic channel registration, channels can be registered after dma
controller registration. Users may want to have specific channel names.

If name is NULL, the channel name relies on previous implementation,
dma<controller_device_id>chan<channel_device_id>.
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20240531150712.2503554-11-amelie.delaunay@foss.st.comSigned-off-by: Vinod Koul <vkoul@kernel.org>