ASoC: WM8804: Use a proper DAI name

Make sure the DAI name does not include a '/' since we might have
per DAI debugfs or sysfs entries in the future.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>