ASoC: ep93xx_pcm: Fix compile error

Commit 453807f3 ("ASoC: ep93xx: Use ep93xx_dma_params instead of
ep93xx_pcm_dma_params") introduced a small compile error by not updating the
name of the 'dma_port' field to 'port'. This patch fixes it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>