tests: drivers: dma: chan_blen: correct nodelabel filter

When attempting to add a new platform and run the suite with
`twister`, it seemed to be resulting in being statically
filtered-out. Likely, the correct filter is `test_dma0`,
assuming that `test_dma*` nodes should be enumerated starting
at 0.

Verified the fix by adding `dma` as `supported` to
`boards/arm64/bcm958402m2_a72/bcm958402m2_a72.yaml` and by
adding the `test_dma0` node to
`chan_blen_transfer/boards/bcm958402m2_a72.overlay`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>