ASoC: fsl-asoc-card: enable dpcm_merged_chan flag for ASRC frontend

ASoC: fsl-asoc-card: enable dpcm_merged_chan flag for ASRC frontend

When using ASRC in DPCM mode, the backend DAI (codec) may have channel
constraints that differ from the frontend. For example, the ASRC can
support 1-8 channels, but the codec might only support stereo (2 channels).

Without dpcm_merged_chan, userspace can open the frontend with unsupported
channel counts, leading to errors when the backend is configured.

Enable dpcm_merged_chan to merge backend channel constraints to the
frontend, ensuring userspace only sees valid channel configurations.

This fixes issues where applications attempt to use channel counts
that the backend codec doesn't support.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://patch.msgid.link/20260429100028.2739711-2-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>