ASoC: twl6040/sdp4430: Change legacy DAI name

Change the legacy DAI name from "twl6040-hifi" to "twl6040-legacy" to
be more intuitive.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>