ASoC: fsl_spdif: change IFACE_PCM to IFACE_MIXER

ASoC: fsl_spdif: change IFACE_PCM to IFACE_MIXER

As the snd_soc_card_get_kcontrol() is updated to use
snd_ctl_find_id_mixer() in
commit 897cc72b0837 ("ASoC: soc-card: Use
snd_ctl_find_id_mixer() instead of open-coding")
which make the iface fix to be IFACE_MIXER.

Fixes: 897cc72b0837 ("ASoC: soc-card: Use snd_ctl_find_id_mixer() instead of open-coding")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://patch.msgid.link/20241126053254.3657344-3-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>