ASoC: fsl-asoc-card: Update the rtd query

sound card rtd was an array and was updated to a list so update
the driver to use a list

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>