dmaengine: pl08x: constify pointer to char in filter function

The opaque argument chan_id passed to filter function is actually
pointer to const memory, so make that obvious in the filter for code
readability and safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240208202742.631307-1-krzysztof.kozlowski@linaro.org


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